Fibre channel arbitrated loop bufferless switch circuitry to increase bandwidth without significant increase in cost

ABSTRACT

A switch, switched architecture and process for transferring data through an FCAL switch is disclosed. The switch uses multiple switch control circuits each coupled to one FCAL network and all connected to a crossbar switch. The switch control circuits are coupled together by a protocol bus for coordination purposes. Local conversations can occur on each FCAL loop and crossing conversations through the switch can occur concurrently. The OPN primitive is used to establish the connection before any data is transferred thereby eliminating the need for buffer memory in the switch control circuits. The destination address of each OPN is used to address a lookup table in each switch control circuit to determine if the destination node is local. If not, the destination is looked up and a connection request made on the protocol bus. If the remote port is not busy, it sends a reply which. causes both ports to establish a data path through the backplane crossbar switch.

FIELD OF USE

This is a divisional of a co-pending patent application entitled FIBRECHANNEL ARBITRATED LOOP BUFFERLESS SWITCH CIRCUITRY TO INCREASEBANDWIDTH WITHOUT SIGNIFICANT INCREASE IN COST, Ser. No. 09/195,846,filed Nov. 19, 1998. The applicants wish to drop their claim to priorityin the parent case under 35 U.S.C. Section 120 to U.S. patentapplication entitled FIBRE CHANNEL LEARNING BRIDGE, LEARNING HALFBRIDGE, AND PROTOCOL, Ser. No. 08/786,891, filed Jan. 23, 1997.

Fibre Channel networks are known loop configuration networks that have aplurality of known type nodes such as servers, printers, disk arraysetc. all connected together by the loop. Such networks use a uniqueprotocol involving a plurality of 40 bit primitives that are used toarbitrate for loop control, to establish connections and to carry outflow control for data transfers of frames of data. The flow controlinherent to the Fibre Channel Arbitrated Loop network (hereafter FCALnets) protocol has the advantage of eliminating the need for the nodesto have extensive buffering capabilities since the destination nodecontrols the amount of data it receives by transmission of an RRDYprimitive to the source node each time the destination node is ready toreceive another frame.

Fibre Channel networks emerged as a family of interconnection topologiesto increase bandwidth over fast-wide SCSI networks and to increase thenumber of server and storage elements that can be connected to 126 overthe 16 device limit of SCSI. Advantages of FCAL include that devices maybe farther apart (up to 10 km) and more numerous and that the size ofdata transfers is very large compared to the overhead that is requiredto set up every transfer. This makes FCAL very efficient and moreattractive than less efficient protocols such as TCP/IP over Ethernetand SCSI over a bus connection.

Hub based network topologies are generally desirable because theyovercome certain limitations on the number of nodes that can be coupledto a network by breaking it up into segments coupled by the hub. ManyEthernet networks use hubs as do token ring networks. Hubs in FCALnetworks receive packets from a source node on an input line coupled tothe source node and rebroadcast the packet on an output line coupled tothe next node which rebroadcasts the packet to the next node and so on.The rebroadcast by subsequent nodes in the chain wastes computingresources. Switched topologies work differently in that packets are notrebroadcast, but instead are connected directly to the line coupled tothe destination node thereby eliminating processing by other nodes whichare not the destination to receive and rebroadcast messages not destinedfor that node.

Despite their advantages, a significant problem in FCAL networks isdelay and this delay increases as the network scales up in size. Eachmeter of cable contributes 5 ns of delay. Further, each node contains anelasticity buffer or FIFO to absorb the differences between incoming andoutgoing data rates. Data passing through a node enroute to itsdestination passes through the nodes elasticity buffer and suffers atypical delay of 3 words. Typically, disk clusters are 10 drives to acluster with each drive being one node and imposing its own delay. Ifthere are 10 clusters coupled to a server, this would representtypically 5.3 microseconds of delay in transition of each primitive anddata frame travelling around the loop. In other words, this delay isimposed on each loop tenancy. In an I/O operation, there are typically 4tenancies for a write to disk, each involving 3 “round trips”: ARB,OPN-RRDY and Data/CLS (see ANSI standard X3T10 FCP which is herebyincorporated by reference). Thus, 12 delays would be suffered by eachcommand transaction. On a 100 node loop, this translates toapproximately 64 microseconds of delay per command.

The command overhead of modem disk drives is around 200 microseconds andfalling. The delay per command coupled to the command overhead of thedrive imposes a significant penalty on performance of approximately 32%.For random access benchmarks with small I/O payloads typical of databasequeries, the performance penalty becomes more pronounced. The problemmanifests itself as the inability of the server to achieve more I/Ooperations per second, regardless of how many more disk drives are addedto the system.

Spatial reuse provided by switches or hubs which allow concurrent looptenancies is one way of reducing the delay problem. The IBM serialstorage architecture in the prior art is one method of providing spatialreuse.

Connection oriented switched topologies were tried in early FibreChannel Fabric networks to attempt to overcome the delay problems ofloops by cutting down the number of nodes each primitive and data framepasses through in getting from source to destination and providingspatial reuse. These early fabric switches were complicated, expensiveand slow, all of these characteristics being found quite undesirable byartisans of FCAL networks. In the early FC Fabric switches, an entireframe of data with a header that indicated the destination node to whichthe frame was directed was sent to the switch for purposes of requestinga connection. These early switch designs had microprocessors which wereused to implement several layers of software architecture to receive theframe, pass it up through various layers of processing to find the frameboundaries, crack the frame open, determine its destination address andthen attempt to find the destination node and make the switchingconnection. The entire frame of data had to be buffered during thisprocess of attempting to find the destination and make the properconnection. It was possible in this early design that the connection wasnever made, because, for example, the destination node was busy withanother conversation. The switch would then have to send a message backto the source that no connection was made and to try again later.Because of limited buffer space in the switch, the data in the originalframe might need to be overwritten by other data from a frame of dataembodying another request. In such a case, the switch would have to sendanother message to the source saying, “Sorry, I lost your data. Executeerror recovery protocol.” Error recovery protocols further complicatedthe operation and contruction of such systems. If a connection is made,the switch receives another frame of data back from the destination.This frame also must be received, have its boundaries detected and mustbe cracked open to examine its contents to see if the destination issaying, “Yes, I am available for a connection.” This type of switchproved to be unworkable and FCAL loops became the standard interconnectfor disks and servers.

Prior art Fibre Channel switches are commercially available from Ancorand Brocade Communications which provide spatial reuse and efficientlink utilization. The FL_ports connected to these switches also addressphysical delays as they pertain to FCAL. However, these switches requirelink rate frame buffering to accomplish their performance levels, andalso operate on the entire 24-bit address contained in the FC frame. Incontrast, the invention described herein uses zero buffering and an8-bit address decode for a much more efficient and inexpensive design.

Many network switched topologies that use entire frames of data torequest a connection through the switch suffer these same drawbacks. Theneed for errory recovery protocols arise because of the potential forlost data arising from the fact that only limited amount of memory canbe put in the switch at realistic costs, and in heavy trafficsituations, the memory may be exhausted and some portion thereof mayhave to be rewritten with new data before the original data isdelivered. Memory is expensive, takes up space and complicates thedesign.

Examples of other network topologies other than Fibre Channel Fabricthat suffer these same drawbacks are the 1 Gigabit Ethernet® and ATMprotocol networks now in public use.

The Fibre Channel Arbitrated Loop (FCAL) topology emerged as a way ofproviding simple, low-cost connectivity to more nodes over a sharedmedia than could be provided in point-to-point topologies without therequirement for an expensive fabric switch. FCAL networks allow up to126 node ports to be coupled by a shared media using a simple protocolwithout the need for a separate fabric switch. Unlike the switchedfabric topology which has a centralized approach to routing, FCALnetworks distribute the routing function to each loop port. This reducesthe cost of achieving interconnection since the loop functionalityrepresents a relatively small addition to the normal port functionalitythat has to be present anyway. However, FCAL networks suffer thedisadvantage that the number of concurrent interconnections possible issmaller than in switched fabric networks because FCAL networks are fullyblocking topologies such that only two pairs of nodes on the loop cancommunicate at any particular time. All other nodes have to wait untilthe first pair are done communicating before the loop is available foranother pair to communicate. The address space is also limited to 126nodes. Another problem with FCAL topologies is that traffic originatingfrom a source node had to travel through each intermediary node on theloop portion between the source node and a destination node. Since eachnode imposed a delay, the overall bandwidth was decreased since eachloop tenancy involved a protocol wherein OPN, RRDY and CLS primitivesand data frames had to travel through all these intermediary nodes inorder to complete the loop tenancy. Since no other pair of nodes couldcommunicate until the loop tenancy was complete, the delays intransmission imposed by each node on each portion of the protocoldecreased overall bandwidth and throughput.

In an attempt to further increase bandwidth and concurrency without theextremely high cost of fabric switches, combinations of FCAL loops withsmaller fabric switches have been devised. This allows the cost per portof the fabric switch to be amortized over the the total number of portsincluding those coupled to the subloops coupled to the switch.

The problem with the approach of coupling multiple FCAL loops togetherby a fabric switch is that each FCAL subloop must be coupled to thefabric switch by a complicated port called an FL_port. These are portswhich must be able to understand the FCAL loop protocol on one side ofthe port and interface it with the very different and substantially morecomplex fabric switch packet switching protocol on the other side of theport while also having bridging functionality. Such FL_ports must haveprotocol layers that understand each protocol and can do packet routingand communicate with each other.

Further, FL_ports are expensive to build. This is because of theinordinate amount of buffer memory that is needed in the front end ofthe FL_port to reconstruct the sequences of packets contained in oneFCAL tenancy. Modern day connectionless fabric switch protocols are purepacket switching while FCAL loop tenancy protocols are singleconversations which tend to be like a simple switched circuit andcontain many packets. That is, the FCAL loop tenancy protocol startswith an OPN primitive directed to a destination node which responds withan RRDY directed to the source. These primitives set up switches in thenodes so that the source and destination nodes talk directly to eachother through the loop segment between them and the intervening nodes bysending one or more frames of data until a CLS primitive is sent whichends the conversation.

In contrast, the packet switching done by an FL_port on the switchbackplane side is not nearly this simple. The packet switching protocolrequires the FCAL frames output by a source node to be treated as manyindividual packets. Those packets must be routed to the correctdestination FL_port and reassembled there without loss of data andlaunched on the FCAL loop on which the destination port is resident.Thus, FL_ports will receive packets from the fabric switch side and dataframes and primitives from the FCAL loop side and must have the memoryand intelligence to convert between the two protocols.

In the treatise Kemble, Arbitrated Loop, Chap. 1, pp. 18-19, FIG. 12,Published by Connectivity Solutions, Tucson, Ariz. (1996), ISBN0-931836-82-4, Kemble proposes a “Smart Hub” conceptual network. Thisnetwork is comprised of a plurality of FCAL loops coupled together by asmart hub which has the intelligence to provide independent operationswithin each loop. When a source node wants to exchange data with adestination node, it arbitrates for its local loop and attempts toestablish a loop connection with the destination. If the destination islocal, the smart hub simply acts as a repeater. If the destination portis not local, the smart hub intercepts the attempt to establish a loopconnection, acquires access to the proper destination loop andestablishes the loop connection between the source and destination loopsacting like a bridge. Thus one loop can talk to another withoutaffecting activity which is purely local to the other loops not involvedin the connection, but the other loops cannot set up simultaneousconnections to nodes on other loops during the tenancy across the smarthub of the first cross-boundary connection between the two loops alreadyconnected across the smart hub. This type arrangement cannot satisfy theneed for a fast switch which is affordable and provides the ability formultiple pairs of ports to communicate simultaneously across the switch.

As a response to the limitations on concurrency present in conventionalFCAL topologies, the assignees of the present invention devised anetwork topology using intelligent hubs each of which had routingintelligence and each of which had its own subloop coupled to aplurality of conventional L_port nodes of a type used in conventionalFCAL topologies. Each hub was coupled to each other hub by a broadcastdata path and a return data path. By watching the addresses in the OPNprimitives and the flow of primitives, the hubs were able to deduce thelocation of the source and destination nodes and cut out all subloopsand nodes thereon that were not necessary for communication between thesource and destination nodes thereby decreasing unnecessary delay incompleting each loop tenancy and increasing bandwidth. Further, someconcurrency was supported in that tenancies between source anddestination nodes on the same subloop could be completed simultaneouslyon each subloop, thereby further increasing throughput. This technologyis described in U.S. Pat. No. 5,751,715 which is hereby incorporated byreference.

The assignee of the invention has also filed a co-pending patentapplication on a learning bridge for FCAL topologies such that two ormore FCAL loops can be coupled together by bridges. The bridges have theintelligence to examine the destination addresses of OPN primitivesreceived from their local loops and watch the primitives of looptenancies and the loops from which they came and to learn the locationsof various nodes on the loops to which they are coupled. Each bridgethen forwards OPN primitives from one loop to another if the destinationnode is on a different loop than the source node, but keeps the OPNlocal to the same loop as the source is on using a local bypass datapath if the destination and source nodes are on the same loop. Thisprovides the ability for concurrent loop tenancies to be occurring onthe two loops coupled to the bridge, although only one loop tenancy perloop is allowed at any particular time. This technology is described inthe parent application incorporated by reference herein.

Despite these improvements over standard FCAL topologies provided by theassignee, there is still a need for further improvements in concurrencywithout the increased cost and increased protocol complexity of fabricswitches. What is needed is a way to achieve the high concurrency andbandwidth of fabric switch topologics without the high cost thereof.Therefore, a need has arisen for a relatively simple, FCAL switch whichhas little or no buffer memory needed therein and which is capable ofestablishing connections very fast. Fundamentally, what the prior art ismissing is a switch which can couple multiple FCAL loops together withhigh concurrency and the speed of a fabric switch that uses N_ports andwhich couples FCAL loops together without the expense, complexity,memory demands and slowness of fabric switches that use FL_ports(FL_ports can support fast switching, but the amount of memory neededmakes the cost prohibitive). This switch, in an ideal world, would havemultiple ports, each of which is coupled to an FCAL loop or an NL nodeand provide concurrency such that each port can talk to any other portat any time the other port is not already tied up in another looptenancy.

SUMMARY OF THE INVENTION

Two important attributes of all species within genus of the inventionare: first, the use of the destination address in an FCAL OPN primitive(hereafter referred to as an OPN) instead of a frame header of a frameof data to find the destination node and establish the connectionthrough the switch; and, second, using the normal flow controlprimitives of the FCAL protocol for hold back purposes to eliminate theneed for large buffer memories in the switch and so as to implement aswitching protocol to stream complete data frames from source todestination without storing any data frames in the switch, and withoutany packetizing, and without any segmentation and reassembly processing,and without any error recovery protocols to retransmit dropped frames.Any species that shares these two characteristics is within thepreferred genus of the invention. This preferred genus qualifies forclass 2 Fibre Channel operation where frames cannot be dropped becausethe flow control nature of the switch prevents frames from ever beingdropped because they are only transmitted when the destination node hasindicated it can receive them. The preferred genus can also be operatedin class 3 Fibre Channel operation where it is permissible to dropframes and upper level protocols do error recovery for dropped frameseven though the switch never drops frames.

A separate second genus of FCAL switches, suitable for Class 3 FibreChannel operation only, still uses the destination address in the OPN tofind the remote port but uses buffers instead of hold back flow controlto complete the transaction to busy remote ports. Specifically, specieswithin this genus will use the destination address of the OPN from thesource node to find the location of the remote port. Then the status ofthat port will be checked. If the status is available, a connectionrequest will cause a connection to be set up between the source node andthe destination node via a source port connected to the source node anda destination port connected to the destination node. The buffer comesinto play when the destination port is busy. In this situation, in thefirst genus described above, the normal primitives of the FCAL protocolare used for flow control to prevent the source node from transmittingany frames of data until the destination port becomes available. In thesecond genus defined in this paragraph, a buffer big enough to hold oneor more complete frames of data is included in the front end of eachswitch chip, or multiple buffers each big enough to store a frame ofdata are included with each switch chip front end. Each of these bufferswill serve as an auxiliary switch port and have its own connection tothe backplane in some species or a single shared connection to thebackplane through a multiplexer can be used. The preferred species usesmultiple buffers each with its own connection to the backplane inaddition to a connection directly from the switch port to the backplanefor direct connections without buffering. In some species, a singleshared buffer or multiple shared buffers on the backplane or in somecentral location may be used.

In this second genus, the way the buffers are used is for the sourceport to generate an RRDY sua sponte when it finds from a check of thescoreboard that the destination port is busy. The RRDY is sent to thesource node and causes it to output a frame of data. This frame of datais stored in the switch port's buffer. Then a message is sent to thedestination port indicating that the auxiliary buffer of the switch portis holding a frame of data for the destination port. This auxiliarybuffer ID is added to the camp list for the destination port. When thedestination port becomes available, a message is sent back on theprotocol bus indicating that the destination port is now available andnaming the backplane channel to use. A connection through the backplaneis then established to this channel by the auxiliary buffer connectioncircuitry and the destination port, and the data in the auxiliary bufferis transmitted. If the switch port has multiple auxiliary buffers, theyeach have their own IDs and, preferably, each has its own switchingcircuitry to make a connection to the backplane.

In this second genus, each auxiliary buffer has circuitry coupled to thereturn path to recognize RRDYs transmitted back by the destination nodeand to count them (or store them) and to wait for a connection betweenthe source port and the RRDY counting circuit if the connection is notcontinuous such as in some cases where multiple buffers are present ineach switch port. These stored RRDYs (or self generated in the case of acount only) can be transmitted to the source node in the case of fullduplex or mixed with frames from a third node in the case of a dualsimplex connection and transmitted to the source node. Each source portalso has shared circuitry for each FCAL net which recognizes incomingRRDYs from the source node and counts them or stores them. These sourcenode generated RRDYs can be transmitted to the destination node in thecase of full duplex or transmitted to a third node in the case of dualsimplex.

Returning to consideration of the first genus, the normal buffer bybuffer accounting and the hold back, handshaking nature of the FCAL loopprotocol with large data frames makes this genus of switches possibleand also very efficient.

The FCAL OPN primitive is a small 40 bit quantity which includes a codeindicating it is an OPN primitive and includes a destination address,and an optional source address if the OPN is full duplex. Receipt of theOPN starts the process carried out by the switch of finding thedestination and causes establishment of the connection or a notificationto the source that the connection could not be established before anydata frame is ever transmitted to the switch. This lack of transmissionof any large data frame before establishment of the connection meansthat the switching circuits connected to each FCAL loop coupled to theoverall switch structure do not need to have buffer memories to storethe data while the connection is being made or the fact that aconnection is not possible is established. This allows for greatlysimplified hardware. This means lower costs and greater density of portsper chip. This cost advantage is a significant improvement over priorart approaches.

The major subclass of embodiments taught herein uses a plurality ofports for connection to individual FCAL loops, and a crossbar switchwhich couples the ports together and which can implement any number ofseparate data transfer channels under control of the ports with the portusing the destination address information in the OPN primitives todetermine whether or not a connection through the backplane from oneport to another is needed. In the preferred species, the ports areintegrated circuits with many ports on one chip and a portion of adistributed crossbar switch also integrated on the chip to selectivelycouple the integrated circuit to one of the backplane data pathchannels. Each port is essentially a learning bridge front end with aninterface to the crossbar switch on the backend.

Fairness is provided, in the preferred embodiment, by a fairness tokenwhich circulates to all the ports and which, when held by a particularport, gives that port “high priority status”. This means that if an OPNcomes in to a port with the fairness token in its possession and thedestination node is on a remote port, the high priority status of thatport means that it can “camp” on the remote port and wait for it to beavailable and it is guaranteed access to the destination node no matterhow busy it is. Since the fairness token circulates, no port will everbe starved from communication with a busy node.

Different variations or species within the subclass are taught.Distinctions between species within the subclass are based upon: the waythe destination node is found; the way in which the first port coupledto the source node signals the second, remote port that there is trafficwaiting for one of the NL nodes to which it is coupled; whether thecrossbar switch is central or distributed; whether the complete routingtable is stored in each port or there is a single separate routingtable, or whether there are partial routing tables stored in each port;whether a scoreboard is used or not to determine the status of a remotenode; and, if a scoreboard is used, whether it is distributed with acopy in each port or centralized and shared by all ports. All thesevariations between species and combinations of variations are equivalentto each other even though each has its own peculiar advantages anddisadvantages.

As an example of variations between species within the inventive genusdefined above consider the following. Location of the destination nodecan be by any of several means since the OPN includes the destinationaddress therein. In one embodiment, the destination address from the OPNis used to address a lookup table which outputs data as to which loopthe destination node is on and to which switch chip or port coupled tothe destination loop the switching connection should be made. Thisembodiment has the advantage that all the connection information isimmediately available. This allows the connection to occur more rapidly.The disadvantage of this species is that the look up table is larger andeach port must carry a full copy of the routing table.

One alternative embodiment uses a destination location process whereinthe destination address of the OPN from the source node is used toaddress a lookup table (hereafter LUT) which only outputs a single bitindicating, in one logic state, that the destination node is “local”,i.e., on the same FCAL network as the source node, or indicating, in theopposite logic state, that the destination node is not on the localloop. If the destination is not local, the destination address isbroadcast as a location request to the other switch chips coupled to theother FCAL networks connected to the switch. Each of the other switchchips then checks its local LUT using the destination address todetermine if it has the destination node on its loop. The switch chipthat has the destination node finds this out from data returned from itslocal LUT and then sends a message to the switch chip coupled to theloop having the source node telling it to where the connection is to bemade and whether the connection can be made, i.e., the loop upon whichthe destination node is not busy in another conversation and isavailable for the connection. The advantage of this species is a smallerrouting lookup table may be used in each port. The disadvantage is therequirement of more message traffic between chips resulting in slowerresponse.

An example of an FCAL switch within the genus of the invention is abufferless switch for coupling to a plurality of FCAL nets and having acrossbar switch and FCAL loop interface port circuits structured to usethe OPN and RRDY primitives of the FCAL protocol for hold back flowcontrol to eliminate the need for a buffer with the ports and crossbarswitch structured to provide multiple simultaneous loop tenancies.

One embodiment for a protocol within the genus of protocols which definethe rules to set up a connection through an FCAL switch within the genusof the invention between a source node and a destination node andtransfer data therebetween is:

-   -   1) in a source node, arbitrating for and winning control of a        first FCAL net and transmitting an OPN primitive thereon, the        OPN primitive having a destination address of a destination node        therein;    -   2) receiving and latching at a first port of an FCAL switch the        OPN primitive from the source node coupled to the first port by        the first FCAL net;    -   3) using the destination address in the OPN primitive as a        search key to search a routing table to find the location of a        destination node having the destination address in the OPN or        the ID of a port coupled by an FCAL net to the destination node,        or both, and, if the destination node is coupled to the first        port, passing the OPN primitive to the destination node via the        first port via a local bypass data path coupling an input of the        first port to an output of the first port coupled to the first        FCAL net, but, if the destination node is coupled to a second        port other than the first port, controlling a crossbar switch to        establish a data path between the first and second ports and        determining if the second port is available, and, if so, sending        the OPN primitive to the second port indicating traffic is        waiting to be sent to said destination node and latching the OPN        in said second port;    -   4) in the second node, arbitrating for control of a second FCAL        net coupled to the second port;    -   5) when control of the second FCAL net is won following said        arbitration, forwarding the OPN to the destination node;    -   6) receiving an RRDY primitive or a CLS primitive from the        destination node in the second port and transmitting the        primitive so received to the source node through a connection        established across crossbar switch, and through the first port        and the first FCAL net; and    -   7) for each RRDY received by the source node, transmitting a        frame of data to the destination node through the first FCAL        net, the first port, the data path through the crossbar switch,        the second port and the second FCAL net without ever storing it        in a buffer in the switch, and continuing to pass data frames        and primitives between the source and destination nodes, until a        CLS primitive is transmitted by either the source node or the        destination node, and then closing the data path through the        crossbar switch and relinquishing control of the first and        second FCAL nets.

The preferred subclass of the switch utilizes the concepts of thelearning bridge taught in the parent application incorporated byreference herein for front end circuitry coupled to the FCAL net withbackend circuitry which is coupled to a crossbar switch. The bridgingfront end uses the destination address in the OPN to decide whether ornot to connect the front end circuitry to the back end circuitry. Thecrossbar switch implements a plurality of completely separate data pathsthrough the switch each of which can couple two ports together. Theprovision of multiple separate data paths through the crossbar switcheliminates any bottlenecks which could occur if a multiplexed data buswere to be substituted for the crossbar switch. It is within the genusof the invention however to substitute a multiplexed bus for thecrossbar switch using any form of multiplexing.

Thus, the switch apparatus genus could be generally described asincluding multiple species, each comprised of a plurality of halfbridges, each with a front end for connnecting to an FCAL loop and abackend coupled to either a crossbar switch or a multplexed bus alongwith suitable control circuitry to use the destination addresses in OPNprimitives to determine whether a connection between two ports throughthe crossbar switch or multiplexed bus is necessary and, if necessary,for establishing the connection.

In the preferred embodiment, each half bridge is one port. In thepreferred embodiment the half bridges are implemented as integratedcircuits with a multiplicity of half bridges on every chip with eachhalf bridge building its own routing table by a passive learningprocess. An alternative embodiment uses an active discovery process tobuild the routing table.

The switch architecture can be thought of as a multi-port switch with astack of learning half bridges substituted for each FL_port of a priorart fabric switch, with each half bridge on each layer being coupled toits own local FCAL loop or single NL node. The other side of each halfbridge is connected to the high speed crossbar switch in the preferredembodiment so that it can be connected to the other half bridges. Thecrossbar switch can be thought of as a stack of separate layers ofseparate high speed backplane data paths connecting all the half bridgestogether by way of a switching network between the high speed backplanedata path layers. The switching network functions to establish selectiveconnections between layers and can be controlled such that any bridge onany layer can talk to any other bridge on any other layer. This allowsmultiple concurrent connections across the switch between a plurality ofpairs of source nodes on one loop and a plurality of pairs ofdestination nodes on other loops or source and destination nodes coupledindividually to half bridges. The switch architecture allowssimultaneous purely local loop tenancies on any FCAL net coupled to anyparticular half bridge so long as another node on the FCAL net is notinvolved in a loop tenancy which involves communication across theswitch from one port on one FCAL net to another port on another FCALnet.

Flow control using the OPN primitive only to establish connectionsacross the switch is used to eliminate the need for large amounts ofmemory. As a result, the switch is capable of operating at a highthroughput rate, but neither the half bridges nor the crossbar switcheshas the amount of memory of an FL_port of a fabric switch that would berequired to make the fabric switch capable of operating at the samethroughput rate.

Another significant advantage of the invention is that the nodes on theindividual FCAL nets can be conventional NL node designs which alreadyexist. An NL node is a node on an FCAL net which understands and canimplement the FCAL flow controlled loop connection protocol betweensource and destination nodes to transfer data using OPN, RRDY and CLSprimitives and large data frames. Because the embodiments of theswitches described herein are all compatible with conventional NL nodes,the genus of switches described herein has the advantage that when thenetwork is upgraded, only the switch needs to be upgraded and all thenodes can remain the same thereby saving substantial expense to thecustomer.

Dual simplex capability is also taught to improve the throughput of anynetwork of FCAL nets coupled by any type of switch. Dual simplexcapability allows a source node on a first FCAL net which istransmitting data on a front channel connection to a destination node ona second FCAL net through a switch connection to simultaneously receivedata via a back channel connection from a third node on a third FCALnet. This is advantageous to improve throughput because in many cases,destination nodes to which data has been transmitted have no data totransmit back to the source node that sent them the data while othernodes do have data to be transmitted to the source node.

Dual simplex capability is accomplished in all species within the genusof the invention to further increase throughput. It is accomplished bythree basic steps, illustrated in FIG. 15:

-   -   1) establishing a front channel half duplex data path between a        source node and destination node on different FCAL nets (step        350) and stripping and storing or counting any buffer credit        RRDY primitives output by the source node and not transmitting        them to the destination node (step 352);    -   2) establishing a back channel data path between a third node        and said source node but not transmitting to said source node        any OPN primitive emitted by said third node (step 354), and        transmitting a number of RRDYs either equal to the number of        RRDYs output by said source node or the number of RRDYs needed        by said third node to send all the data it has to said source        node before closing said back channel connection (step 356),        transmission of said RRDYs being one at a time—any excess RRDYs        not used by the third node are saved for use by another third        node in a subsequent dual simplex back channel connection; and    -   3) receiving any RRDYs transmitted by said destination node and        mixing them in with data frames and/or primitives transmitted on        the back channel by the third node so as to exercise flow        control on transmissions of data frames from the source node to        said destination node (step 358).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of one example of an FCAL switched architectureaccording to the teachings of the invention.

FIG. 2 is a drawing of another example of an FCAL switched architectureaccording to the teachings of the invention having multiple subloopswithin some of the FCAL coupled to the switch.

FIG. 3 is a block diagram of the preferred switched FCAL architecture.

FIG. 4 is a flow chart of the processing which occurs for a purely localtransaction.

FIG. 4 is a block diagram of the general preferred architecture of aswitch to couple a plurality of FCAL nets to provide spatial reuse.

FIG. 5 is a block diagram of the specific preferred “switch slice”architecture of a switch to couple a plurality of FCAL nets to providespatial reuse with multiple port circuits and a portion of the crossbarswitch integrated on each switch chip.

FIGS. 6A through 6C are a flow diagram of a routing algorithm forgeneral switch mode operation (non dual simplex).

FIG. 7 is a block diagram of the preferred architecture of each switchchip in the FCAL switch system.

FIG. 8 is a diagram of an FCAL switch with two switch chips configuredto run in hub mode.

FIG. 9 is a diagram of an FCAL switch system with two switch chipsconfigured to run in switch mode and illustrating loop-local, chip-localand remote port simultaneous loop tenancies.

FIG. 10 is a block diagram of the multiplexer structure in each switchchip that allows port bypass mode, parallel loopback mode and serialloopback mode to be implemented.

FIG. 11 is a table describing and naming each state in the loop portstate machines of each port of each switch chip.

FIG. 12 is a table of the source port fill word generation for variousinput words and states.

FIG. 13 is a table of the destination port fill word generation forvarious input words and states.

FIGS. 14A through 14E are diagrams of the five different message formatson the protocol bus.

FIG. 15 is a generic flow diagram illustrating the minimum basic stepseach species in the genus of the invention would have to carry out toimplement dual simplex operation.

FIGS. 16A through 16D are a flow chart of the specific steps carried outby the preferred embodiment to carry out dual simplex communications.

FIGS. 17 through 28 (each comprised of multiple figures, such as FIG.17A and FIG. 17B, which should be pieced together at the cut line) areindividual state diagrams for all the state machines in a single LoopPort State Machine (LPSM).

FIG. 29 is a block diagram of a species of a buffered FCAL switch whichfalls within a separate second genus of FCAL switches, suitable forClass 3 Fibre Channel operation only.

DETAILED DESCRIPTION OF THE PREFERRED AND ALTERNATIVE EMBODIMENTS

The published Fibre Channel Arbitrated Loop standards memorialized inthe following ANSI standards are hereby incorporated by reference:X3.230-1994 describing the physical and signalling interface;X3.297-1996 describing the physical and signalling protocol; X3.272-1996describing the general FCAL protocol and TR-20-199X, T11 Project1235-DT, Fibre Channel Fabric Loop Attachment (FC-FLA).

Referring to FIG. 1, there is shown one embodiment of a switched FCALarchitecture. FCAL switch 10 is coupled to four FCAL networks (hereaftersometimes referred to as FCAL nets) 12, 14, 16 and 18 in this example.Each FCAL net can have one or more NL nodes thereon. Each of the fourFCAL networks is coupled to a plurality of NL nodes which haveconventional structure and which can carry out FCAL arbitration, datatransfer and flow control operations on the FCAL networks. Each node isassigned an address from one of the 127 possible FCAL addresses.

In FIG. 1, each node is given a number symbolizing its addressdesignated in the figure by N and a number inside the circle symbolizingthe node.

The function of the switch 10 is to increase total throughput of thesystem by allowing concurrent conversations to be occurring betweenpairs of NL nodes, and by doing so in a manner that is not limited byany restriction against “crossing conversations” in the switch itself. Anew standard for FCAL topology proposed by IBM involves counterrotating,separate FCAL rings with nodes coupled to both counterrotating rings.This allows concurrent conversations to occur, but the conversationscannot “cross”, i.e., it is illegal in this protocol for bothconversations to require the same segment of an FCAL as part of the datapath for the conversation. “Conversation”, as that term is used herein,means a data transfer between two different nodes. The function of theswitch 10 according to the teachings of the invention is to allow asmany concurrent conversations as possible except that no two sourcenodes can be talking to different destination nodes on the same FCALnetwork. In other words, each of FCAL networks 12, 14, 16 and 18 islimited to only one conversation at a time even though data flow frommultiple conversations may be simultaneously be flowing through switch10. This is done by establishing “virtual channels” for eachconversation through the switch using separate data paths (ormultiplexing techniques in some embodiments). The physical configurationof the switch 10 is not currently believed to be important so long as ituses the destination address in each OPN from a source node to controlsetting up a separate data path through the switch for transfer of databetween that source node and a destination node and uses flow controlprimitives of the FCAL protocol to control the flow of data such thatthe switch does not need to have a buffer memory big enough to hold anentire FCAL frame.

The fact that only 127 addresses are possible is an inherent limitationof the FCAL protocol but it is also an advantage in the following way.Because there are only 127 possible addresses, no microprocessor isneeded in the switch. With a manageable address space, the location ofthe destination node can be determined by looking up the destinationaddresses using a state machine and a lookup table in each switchcontrol circuit coupled to an FCAL network. The lack of a microprocessorboth makes the switch faster and cheaper.

In the topology of FIG. 1, each FCAL network 12, 14, 16 and 18 has a oneGigabit/second data throughput capacity. Therefore, the maximumthroughput of the system shown would be 4 Gigabits/second if each of theFCAL networks 12, 14, 16 and 18 had a purely local conversationoccurring thereon.

One way that the FCAL switched architecture according to the teachingsof the invention can increase throughput is to allow multiple localconversations to occur on each FCAL network through use of bridges. Anexample of a topology that can take advantage of this feature is shownin FIG. 2. There, switch 10 is coupled to FCAL networks 12, 14, 16 and18. However, FCAL network 12 is divided into two FCAL subnetworks 12 and20 by FCAL bridge 22, and FCAL network 14 is divided into two FCALsubnetworks 14 and 24 by FCAL bridge 26. Likewise, FCAL network 16 isdivided into three FCAL subnetworks 16, 28 and 30 by FCAL bridges 32 and34. FCAL bridges 22, 26, 34 and 32 can have the construction detailed inco-pending U.S. patent application entitled FIBRE CHANNEL LEARNINGBRIDGE, LEARNING HALF BRIDGE, AND PROTOCOL, Ser. No. 08/786,891, FiledJan. 23, 1997, the contents of which are hereby incorporated byreference. The maximum throughput in the topology of FIG. 2 would occurwhen each subloop was having a purely local conversation. In thetopology of FIG. 2 with 8 subloops, the maximum throughput would be 8Gigabits/second.

FIG. 3 is a high level block diagram of the internals of the FCAL switch10 for the preferred embodiment using a crossbar switch 44 to make theswitched electrical connections between FCAL networks 12, 14, 16 and 18.Each of the FCAL networks 12, 14, 16 and 18 is coupled to its own switchcontrol circuit, i.e., switch control circuits 36, 38, 40 and 42,respectively. The switch control circuits 36, 38, 40 and 42 aredistinguished from switch control circuits of prior art switch designsby the fact that none of them have buffer memory therein large enough tostore an entire frame of data or enough memory to carry out thepacketization process of prior art switch designs.

The function of the switch control circuits 36, 38, 40 and 42 is totransmit primitives and data involved in FCAL arbitration, data transferand flow control to the appropriate loop segment, do bypass switchingwhen a conversation is completely local so as to bypass the crossbarswitch 44, to collectively locate the destination nodes when OPNs arereceived, and to send appropriate control signals to the crossbar switchonce the destination node has been located so as to connect theappropriate FCAL networks together to complete the conversation.

The crossbar switch 44 has 4 inputs and 4 outputs with 1 input and 1output for each FCAL. The crossbar switch serves to make a data paththrough the appropriate switch control circuit to the input and outputof one FCAL having the source node thereon to the output and input,respectively of another FCAL having the destination node thereon throughthe appropriate switch control circuit so as to provide a channelthrough which the conversation between the source node and thedestination node may proceed. The crossbar switch must be able tosimultaneously connect the input and output of another FCAL havinganother source node thereon to the output and input, respectively of yetanother FCAL having another destination node thereon to provide a datapath or channel for a second concurrent conversation. This concept isextended for as many pairs of FCAL as are connected to the switch. Theparticular connections that are made are controlled by enable signals oncontrol buses 46, 48, 50 and 52. These enable signals are generated bythe switch control circuits based upon the locations of the destinationnodes for the concurrent conversations. Any crossbar switch that canperform the above described function will suffice providing it cansupport the necessary data rate and traffic volume.

The switch control circuits 36, 38, 40 and 42 are coupled by a protocolbus 54. In some embodiments, this protocol bus may be multiplexed usingany multiplexing scheme such as TDMA. In other embodiments, crossbarswitch 44 can be omitted and a TDMA bus substituted with timeslotassignments taking the place of assignments of particular backplanechannels through the crossbar switch and messages on the protocol busupdating all switch ports with information about which timeslots are inuse and which timeslots are available.

Referring to FIG. 4, there is shown a block diagram of the generalarchitecture of the preferred Fibre Channel switch. The switch iscomprised of a crossbar switch 100 coupled to a plurality of learninghalf bridges such as are represented typically by blocks 102, 104 and106. Each learning half bridge has a port having an input and an outputfor coupling to separate input and output wires or fibers of the FibreChannel link. The Fibre Channel link of each port can be coupled to anindividual NL port such as block 108 or an FCAL net such as isrepresented by NL ports 110 and 112 and links 114, 116 and 118. An FCALnet can have one or more NL nodes on it, so even the connection to thesingle node 108 is an FCAL net using a Fibre Channel protocol as thoseterms are used in the claims.

Each learning half bridge in the switch can have a similar front endstructure and mode of operation as that described in the parentapplication Ser. No. 08/786,891, filed Jan. 23, 1997 which isincorporated by reference herein, but preferably has the structuredescribed below in FIG. 7. The “front end” structure refers to thecircuitry that is coupled to the port and any Fibre Channel linkconnected thereto. Each learning bridge port circuit (hereaftersometimes referred to as a port) is coupled to a 50 Mhz 24 bit protocolbus 121. Link 123 represents this connection between port 106 and theprotocol bus. The protocol bus is where each port posts its transactionsindicating its local FCAL net is busy or has become available so as toupdate that port's status information in a local copy of scoreboardtable 125 in memory. In the preferred embodiment, each port maintains asynchronized local copy of the scoreboard table and the contents arewritten to the scoreboard by learning from messages posted on theprotocol bus. “Synchronized” means all copies of the scoreboard have thesame information at all times to avoid “fatal embrace” scenarios (fatalembraces are discussed in the parent bridge case).

In the alternative embodiment of FIG. 4, the scoreboard 125 and routingtable 127 are shown as central shared circuits, but in the preferredembodiment of FIG. 5, every port circuit 124, 126 and 128 has its owncopy of the scoreboard table and a routing table. The protocol bus isalso the communication path used by a first port when an OPN to a remotedestination node comes in to send a request for a connection to a secondport coupled to the destination node. The connection request is sent onthe protocol bus after the first port checks the scoreboard anddetermines that the second port is available. The request causes thesecond port to begin arbitrating for control of its local FCAL net. Thescoreboard contains circuitry to read messages on said protocol bus anduse the information therein regarding the identity of the sending portand the status codes in the message to update the status entry for thesending port. The structure of this circuitry is not critical, andanything that can perform this function will suffice.

Each learning half bridge includes a streaming back end which drives abidirectional port such as ports 120 and 122 coupled to the crossbarswitch. This streaming backend allows large strings of data to be sentall the way from the source node to the destination node as a streamwithout the need to buffer any of it. The structure of the streamingbackend and the half bridge front end is such that hold back flowcontrol is used so that no frames are stored—they just stream all theway from the source to the destination under buffer by buffer accountingusing primitives for buffer management in the nodes themselves and notin the switch. As a result, no frames are ever dropped and there is noneed for buffer management in the switch itself nor any need for droppedframe error recovery protocols in the switch or nodes.

In sharp contrast, in the connectionless fabric switches of the priorart for both FCAL nets and other protocols, the conventional wisdom isthat the essence of the problem is in buffer management. It is actuallypossibie in the prior art fabric switches in the FCAL net environment todrop frames because of congestion related to traffic problems. When aframe is dropped, a complex error recovery protocol needs to be executedin the nodes (node can mean computer, disk drive or other computingmachine coupled to the network) involved to retransmit the droppedframe, or the entire I/O transaction. The error recovery protocol iscostly in terms of overhead, and many I/O applications on the nodes arenot designed to gracefully handle dropped frames thereby creating thepossibility of errors and increasing the complexity of the nodes inrequiring error recovery protocol software to exist in the node. Theacknowledged, connectionless class of service in Fibre Channel (Class 2)does not eliminate or reduce the frequency of dropped frames. the onlyadvantage is that class 2 has negative acknowledgements to the sourcenode from the fabric or destination node if/when any frames are dropped.The only advantage is that this negative acknowledgement may providemore timely notification of dropped frames. It does not improve theintrinsic reliability of FC networks.

In the invention, the hold back flow control which is part of the FCALprotocol is used to advantage to eliminate the need for buffer memory inthe switch. Thus, in the invention, frames are not held in buffermemory, so they cannot ever be dropped because of congestion. The “holdback flow control” used by the invention prevents data from ever beingtransmitted from the source until the switch is ready to stream it toits destination. Therefore, there is no need for any complicated errorrecovery protocols in the switch or in the nodes and no need for massiveamounts of memory in the switch. This is only possible in the FibreChannel protocol where the possibility of using hold back flow controlexists by virtue of the use of primitives and buffer by bufferaccounting. There is no low level flow control in ATM or 1 GB Ethernetprotocols. Those protocols have X-on and X-off flow control on a higherlevel of the ISO level. This X-on and X-off flow control is “embedded”in the data characters, in contrast to Fibre Channel wherebuffer-to-buffer flow control is external to any data or frames. InFCAL, the OPN primitive signals when a source node has data to send, andthe destination node then signals with a RRDY primitive sent back to thesource saying it has reserved space to receive a frame. One and only oneframe is then sent and no other frames are sent until the sourcereceives another RRDY primitive from the destination node. This use ofprimitives and reserved space and buffer by buffer accounting is themeaning of hold back flow control which is used in the invention.

The lack of buffer memory in the half bridges makes them highly compactand susceptible to full integration since memory is a large consumer ofdie area. The ability to integrate multiple half bridges on a singleintegrated circuit die reduces the cost per port to a substantiallylower cost than for a corresponding number of FL_ports.

Prior art switch designs typically had a crossbar switch chip coupled toa plurality of switch module chips each of which had a substantialamount of the die area consumed by buffer memory cells. Thus twoseparate chip designs were necessary to implement a switch. In thearchitecture of the invention since part of the crossbar switch and theport circuitry for several ports are on one IC die, only one chip designis necessary to implement a switch. The various portions of thedistributed crossbar switch on different chips, when coupled together,form one complete crossbar switch. This crossbar switch has 14 backplanechannels, but they are not all available because each switch port chipalso includes access circuitry to the backplane channels. If only twoswitch port chips having the architecture of FIG. 7 are coupledtogether, the amount of backplane access circuitry present is not enoughto access all 14 backplane channels. The number of backplane channelsthat can be accessed with the available access circuitry defines aconcept of “available bandwidth”. As the number of switch port chipscoupled together grows, the amount of available backplane accesscircuitry grows so that the number of backplane channels availablegrows, i.e., the available bandwidth grows which is as it should bebecause traffice volumen is increasing with an increasing number ofports. In other words, each switch port chip is a network slice and anetwork can be fabricated with as many ports as needed, and the crossbarswitch will automatically grow in available bandwidth in proportion tothe number of ports adequately to handle the increased traffic.

FIG. 5 illustrates the actual architecture of the preferred species ofthe separate channels backplane subclass of switches. Each block 124,126 and 128 represents an integrated switch chip having a plurality ofports (learning half bridges) and a portion of the crossbar switchthereon. The function of each of the half bridge ports is to provideswitching capability between networks connected using the Fibre Channelphysical layer and the Fibre Channel Arbitrated Loop transmission layerprotocol. A point-to-point backplane data path 130 comprised of aplurality of channels through the distributed crossbar switch carriesdata from port to port. Each channel has a conductor in both directions.Each channel is a separate one gigabit per second data path which isselectively coupled by the crossbar switch to the two ports involved ina loop tenancy and is not shared by any other ports at that time. In oneexample of an architecture in accordance with the preferred embodiment,there are 14 switchable channels for a total of 28 Gbits/sec ofbandwidth. The function of the backplane data path 130 combined with thecrossbar switch is to carry multiple conversations of data between portssimultaneously.

Since each of the switch chips 124, 126 and 128 contains a plurality ofindependent ports, spatial reuse and concurrency is achieved in thearchitecture of the invention in three ways: first, any purely localconversations between source and destination nodes on the same FCAL netcan simultaneously occur on all FCAL nets without consuming eitherswitch chip or backplane bandwidth; second, any conversations betweendifferent FCAL nets coupled to the same switch chip can occur purelywithin each switch chip without consuming any bandwidth on the backplanebus; and, third, the multiple backplane data paths allow multiplesimultaneous conversations between nodes on FCAL nets coupled todifferent switch chips. For a first example, with 36 ports coupled to 36FCAL nets, a peak achievable bandwidth of 72 Gbits/sec is achievablewith no backplane channel utilization representing 36 simultaneous localfull-duplex conversations. The peak bandwidth through the switch isequal to 2 Gbits/sec per backplane channel (1 GB/sec send and 1 GB/secreceive), plus an additional 2 Gbits/sec for each purely localconversation that can be simultaneously serviced. Thus, with 14backplane channels all in use and 36 ports coupled to 36 FCAL nets withall ports not coupled to one of the 14 backplane channels having localconversations ongoing, the total traffic volume is 28 GB/sec through theswitch plus 22×2 GB/sec equalling 44 GB/sec for a total of 28+44=72GB/sec.

In an exemplary embodiment of an FCAL switch using the teachings of theinvention, each switch chip has 3 ports and there are 12 switch chipsfor a total of 36 ports in this embodiment. Each switch chip supports 14backplane channels such that up to 14 remote full-duplex conversationsthrough the switch 136 can be supported by 28 ports coupled to the 14backplane data paths plus 8 purely local full-duplex conversationsbetween the remaining 8 ports for an aggregate peak bandwidth of 44Gbits/sec. Port and backplane contention will reduce the total bandwidthbelow this peak, but average throughput and arbitration latency of aswitched system will still be faster than an arbitrated FCAL net of thesame size. Spatial reuse provided by a switch allows applications whichmake use of concurrent conversations to use FCAL local loops and aswitch to overcome the one loop tenancy at a time limitation of pureFibre Channel Arbitrated Loop networks without a switch.

Each switch chip is coupled to a fairness token bus 99, which, in thepreferred embodiment, takes the form of a two wire bus. A fairness tokencirculates to all ports on this bus enabling a “round robin” fairnessalgorithm. Each port has circuitry to receive the fairness token, holdit for a short time and forward it to the next port so that the tokenreaches all ports eventually. Each port has a priority level assigned toit that is used in arbitration within the switch of multiple accessrequests to a busy destination node. When the fairness token is in thepossession of a port, that port has the highest priority level and isguaranteed access. When a port has the highest priority level, it can“camp” on a busy remote port, and be guaranteed that it will be givenaccess when the port is available. When access has been granted, thetoken is forwarded. This prevents starvation of any port fromcommunication with a busy node.

In the preferred embodiment, data path 130 is comprised of metallic datapaths on the printed circuit board on which the port chips are mountedcouples the outputs of each crossbar switch portion of one of the portchips to the input of the crossbar switch portion of another port chip.Each bridge portion has three inputs and three outputs (one input andoutput for each of the three half bridges on each chip) for FCAL netprimitives and frame data. Each pair of one input and one output iscalled a port and can be coupled to a single NL node or an FCAL net.

Note that the distributed nature of the crossbar switch and the fullintegration of the ports with the crossbar switch makes the architectureof the switch modular and easily expandable. That is, the architecturerepresents a sliceable architecture which can be built into as large aswitch with as many ports as is necessary limited only by the maximumnumber of permissible addresses in the FCAL address space of 128 nodes.Integration of the ports with a crossbar switch slice on the same chipallows switches to be built with as few as two switch chips or many morebecause the crossbar switch is scaleable and because each chip can becoupled by its portion of the crossbar switch to any one of theplurality of separate backplane channels. Prior art fabric switches hadseparate, nonscaleable circuits for the crossbar switch. This meant thatthe crossbar switch had to be built as complex as was necessary tohandle up to the maximum number of ports. If fewer than the maximumnumber of ports were used, a large part of the prior art crossbar switchcapacity was wasted. In contrast, in the invention, because of the fullintegration of all necessary circuitry onto one chip including a portionof the crossbar switch, the crossbar switch is scaleable, and all sizesof switches are easily constructed and quite affordable because of theinexpensive, fully integrated construction with no buffer memory and nocomplicated error recovery protocols.

Dual Simplex

FCAL NL nodes are capable of duplex communications in that they can senddata at the same time they are receiving data. The problem thatconventional FCAL nets and fabric FL port based switches fail to addressis the fact that typically the destination node in more highly populatedFCAL nets is less likely to have data for the source node but othernodes may have data for the source node which they cannot send since thesource node is tied up in a conversation with the destination node.Thus, there is a waste of bandwidth. Typically, node adapter cards thatinterface a computer or disk drive to the FCAL net cannot re-shuffletheir transmit queues in order to find a frame destined for the currentsource node and move it to the top of the transmit queue. Thus, head endblocking usually occurs and causes a full duplex connection to beeffectively simplex. In a 100 mbyte/sec link for example, NL nodes cansimultaneously burst out at 100 mbyte/sec while they are simultaneouslybursting in at the same rate. If the destination node has no data tosend to the source node, the 100 mbyte/sec of burst in bandwidth of thesource node is wasted in conventional fabric switches with FL ports.

In conventional FCAL nets, the protocol requires that nodes must mustreserve all resources required for a duplex conversation even in caseswhere there is only traffic in one direction. This means, in the contextof FIG. 5, node 101 can be sending data to node 103 but node 103 isrequired by the rules of the protocol to be sending data only to node101 during the same loop tenancy. This is true even though it may haveno data to send to node 101 and even though it may have data to send tonode 105. Likewise, if the switch of FIG. 5 followed the normal FCALprotocol, if node 138 had frames to send to node 103, node 103 would berestricted to sending data only to node 138 even if node 103 had no datafor node 138 but did have data for node 140.

This waste of bandwidth is remedied by the provision of dual simplexconversations across the switch of the invention. The ports and crossbarswitch of the invention allow dual simplex conversations across theswitch but not within any local loop coupled to a port. Simply put, dualsimplex capability allows nodes other than the destination node to senddata frames to the source node while that source node is sending data toits destination node. In the example of FIG. 5, a dual simplexconnection for two separate one-way conversations could occur asfollows. Suppose, node 138 has data to send to node 103 but node 103does not have any data for node 138. Suppose also, node 144 has data fornode 138. With the dual simplex capability of the invention, node 138can send its data to node 103 while simultaneously receiving data fromnode 144.

Dual simplex capability is accomplished in all species within the genusof the invention by three basic steps, illustrated in FIG. 15:

-   -   1) establishing a front channel half duplex data path between a        source node and destination node on different FCAL nets (step        350) and stripping and storing or counting any buffer credit        RRDY primitives output by the source node and not transmitting        them to the destination node (step 352);    -   2) establishing a back channel data path between a third node        and said source node if said source node is dual simplex capable        but not transmitting to said source node any OPN primitive        emitted by said third node (step 354), and transmitting a number        of RRDYs either equal to the number of RRDYs output by said        source node or the number of RRDYs needed by said third node to        send all the data it has to said source node before closing said        back channel connection (step 356)—transmission of said RRDYs        being one at a time—any excess RRDYs not used by the third node        are saved for use by another third node in a subsequent dual        simplex back channel connection; and    -   3) receiving any RRDYs transmitted by said destination node and        mixing them in with data frames and/or primitives transmitted on        the back channel by the third node so as to exercise flow        control on transmissions of data frames from the source node to        said destination node (step 358).        In the preferred species of switch within the genus, the        following steps are carried out to implement dual simplex data        exchanges:    -   1) in a port of the switch coupled to an FCAL net, receiving a        full duplex OPN(Dest, Src) from a source node designating a        destination node address, Dest, and a source node address, Src,        and converting the full duplex format OPN(Dest, Src) to a        simplex or half duplex format OPN(Dest, Dest) and establishing a        “front channel” connection through the crossbar switch to the        destination node (in the preferred embodiment, conversion from        full to half occurs if and only if a configuration bit is set in        each port to allow dual simplex, and, if set to allow it, always        converts all full duplex OPNs to half duplex OPNs even if no        dual simplex connections are expected—in alternative        embodiments, conversion from full to half duplex always occurs        and is not configurable and in other embodiments, dual simplex        is never allowed);    -   2) storing at least the source node address in the port of the        switch coupled to the source node, hereafter referred to as the        source port;    -   3) stripping buffer credit RRDYs output by the source node and        not transmitting the RRDYs output by the source node to the        destination node of the front channel connection, and either        storing or counting the RRDYs output by the source node and        making them available to a third node to send data to the source        node either by transmitting stored RRDYs output by the source        node one at a time to the third node or by conveying the number        of RRDYs output by the source node to a third port coupled to        the third node and synthesizing in the third node a number of        RRDYs equal to the value of the count and sending the RRDYs to        the third node one at a time (the preferred embodiment does not        store the RRDYs—it just counts them and regenerates them in the        source port when needed for transmission to a third node);    -   4) updating status data in a “scoreboard” memory of each port to        indicate which busy ports are coupled to source nodes of front        channel connections and thus are available to receive frames in        dual simplex communication from third nodes—the scoreboard        memory is a memory that stores status data such as whether or        not a port is busy, and, if busy, whether it is coupled to the        source node of the conversation it is engaged in and is thus        open to receive data from a third node in a dual simplex        communication;    -   5) establishing a “back channel” connection through the crossbar        switch from the third node to the source node and transmitting        an OPN from the third node to the source switch port—then the        stored source node address in the source port is compared to the        destination node address of the OPN transmitted from the third        node and allowing dual simplex communication to proceed on the        back channel if the addresses match, but blocking dual simplex        communication with this particular third node if the addresses        do not match by sending a CLS primitive to the third node;    -   6) in the event dual simplex communication is allowed, deleting        the OPN from the third node and never transmitting it to the        source node, and releasing stored RRDYs to the third node        (either one at a time or all at once) thereby allowing the third        node to start sending data frames to the source node; and    -   7) thereafter data frames are transmitted by the third node to        the source port with one data frame transmitted in response to        each RRDY received by the third node—these data frames are mixed        with RRDYs received from the destination node via the front        channel connection—the combined data frames and RRDYs are        transmitted to the source node via the back channel connection.

The “back channel” is the data path going into the source node alongwith a connection through the switch ports and backplane if necessary tocouple the third node to the source node and is the channel used by athird node to send dual simplex data frames into the source node.

The format conversion mentioned above converts the OPN(Dest, Src)received from the source node 138 into an OPN(Dest, Dest). Thisconversion occurs as the OPN is propagated through the source portbefore it is sent to the remote port coupled to the destination node.Conversion of the OPN primitive format to OPN(Dest, Dest) and takingaway the buffer credits from the destination node converts the normalfull duplex loop tenancy to a half duplex or simplex loop tenancy wheredata flows in only one direction from the source node to the destinationnode and the destination node cannot send any frames back to the source.

Borrowing the buffer credit transmitted by the source node and giving itto another node that has data to send to the source node is important todual simplex capability. In the specific example being considered, itmeans that any RRDY primitives output by source node 138 that wouldotherwise give destination node 103 one or more buffers of credit forreturn data are, instead, stripped by the source port before the OPN istransmitted to the remote port, and stored so that they may be given tonode 144 after a simplex back channel connection is established.Specifically, suppose source node 138 transmitted OPN(103, 138) RRDYRRDY. Port 126 would convert this to OPN(103, 103) and transmit it toswitch port 128 when the connection through the backplane between switchports 126 and 128 has been established in any of the ways describedherein. Port 126 would latch the two RRDYs or increment a counter ofbuffer credits to a count of two and give them to node 144 one by oneafter a back channel connection was established between node 144 andnode 138.

Swallowing or deleting the back channel OPN from the third node in thesource port is important to dual simplex because it prevents the sourcenode from receiving it and becoming confused. Because of the structureof the FCAL protocol, it is a protocol violation if a node receives anOPN, and that node has already established a previous connection.However, when a node has opened another node, it is expecting possibledata frames to be sent back to it from the destination node on the backchannel (the part of the loop not being used for outgoing data from thesource to the destination). Thus, if a node other than the destinationnode has frames to send to the source node, the following things happen.First, the third node sends an OPN. The port coupled to the third nodeuses the destination address in the OPN to look up the ID of the portcoupled to the destination node. This search results in output from therouting table of the ID of the source port coupled to the source node inthe original forward channel transmission. This ID is then used tosearch the scoreboard memory to determine the status of the source port.The source port sent a message to the scoreboard memory on the protocolbus updating the status entry in the scoreboard memory for the sourceport to busy when the source port established the front channelconnection. In one alternative embodiment, this busy status will befound when the scoreboard is consulted, but that will not deter thethird port from posting a connection request message on the protocol busrequesting connection to the source port. The source port will determineif it has any stored buffer credit and grant the request if it does byposting a reply message on the protocol bus naming the backplane channelto use. The third port and the source port then both connect to thenamed backplane channel.

There are several alternative embodiments for establishing the backchannel simplex connection through the backplane. One involves updatingall the scoreboards of all ports with information as to which sourcenode address is coupled to any port which is indicated in the scoreboardas having a busy status and is thus available for dual simplex. In thisalternative embodiment, the source port posts a message to the protocolbus to update all scoreboards in every port to indicate that although itis busy, it is coupled to the source node of the loop tenancy and isthus available to receive data in a dual simplex mode. In thisalternative embodiment, the third port checks its scoreboard, and if itdetermines that its destination node has the same address as the sourcenode coupled to the source port, it then sends a connection requestmessage. If it determines from the scoreboard data that the source portis not dual simplex capable, it does not send a connection requestmessage.

In the preferred embodiment each switch port which utilizes dual simplexis only coupled to one node, because the switch ports are not expensive.In this preferred embodiment, the scoreboards are not updated with dataindicating the address of the node connected to the source port so assave memory space in the scoreboards and traffic on the protocol bus. Inthe preferred embodiment, the third node simply transmits a connectrequest on the protocol bus naming the source port. The source port thendetermines if it is dual simplex capable by checking its own scoreboardfor data indicating whether it is dual simplex capable and determines ifits camp list is full. If it dual simplex capable and its camp list isnot full, it grants all connection requests. It then picks a backplanechannel for the back channel connection and sends a response message onthe protocol bus to the third node. Both the third node and the sourcenode then establish a connection on the identified channel and the thirdnode sends its OPN. The destination address in the OPN is then comparedby the source port to the address of the source node using the latchedOPN received from the source node.

If there is no match, the source port generates a CLS and sends it tothe third port. If there is a match, dual simplex mode is allowed, theOPN from the third port is quashed, and the stored RRDYs are sent to thethird port from the source port, one RRDY at a time up to the number ofRRDYs stored. In some alternative embodiments, the RRDYs are notactually stored but are counted and the count is maintained. This startsthe transmission of data frames from the third node. The arrival of adata frame from the third node triggers release of another RRDY from thesource port if another RRDY is available at the source port for release.These data frames are received by the source port and transmitted on theback channel to the source node. Any intervening nodes pass the dataframes through because they do not control the loop. Any subsequentRRDYs output by the source node are intercepted by the source port andtransmitted on the backplane channel to the third port to cause thethird node to transmit a data frame for each RRDY so intercepted. When aCLS is received from the third node, any remaining stored RRDYs arepreserved for use as buffer credit for the next third node that wishesto send data to the source node. The job of managing buffer credit fallsto the state machine in every port in the preferred embodiment.

Another embodiment for the source port to determine if it is dualsimplex capable is to send post a message on the protocol businstructing any port that has an OPN send the destination address of theOPN to the source port for comparison to the source node address. Ifthere is a match, a reply message is posted directing the port to use aspecified backplane channel and both the third port and the source portconnect to that channel. The transaction then proceeds as above.

The dual simplex process essentially tricks the link layer of thesoftware of the source node into thinking the incoming frames are fromthe destination node, when they are actually from the third node. Sincethe frames internally contain all the information needed to get the datatherein to the right place in the source node, no harm is done becausethe frames pass through the link layer and the upper layers of softwareread the frames and use them properly. However if an OPN from the thirdnode were to arrive on the back channel, the link layer of conventionalNL nodes becomes confused because it knows the source node just openedsome other node and is not supposed to be receiving any OPNs until thecurrent loop tenancy is terminated.

The source port also mixes these frames of data from the third node inwith RRDY primitives transmitted from the destination node of theoriginal loop tenancy on the FCAL net back channel. Each time the sourcenode receives an RRDY from the destination node, it outputs anotherframe of data to its destination node. The mixing in of RRDYs from thedestination node with the frames of data from the third node on the FCALnet backchannel causes the source node to continue outputting dataframes bound for the destination node.

Closing a dual simplex connection properly to avoid deadlock andsequence errors is important. The state machine of any port coupled to asource node and facilitating a dual simplex transaction (elsewhereherein sometimes referred to as a dual simplex port or source port) mustinsure that when a CLS is output by the source node that the CLS is notforwarded to the destination node until the third node has finishedtransmissions on the back channel and output its own CLS. Further, thestate machine of any port coupled to a source node and facilitating adual simplex transaction must not wait for a CLS in the case whereaccess to the source node is denied to a third node because of the lackof any RRDYs transmitted by the source node to avoid possible deadlock.Thus, the state machine in a dual simplex port must therefore have thefollowing behaviors as illustrated in the following discussion of fourpossible dual simplex close scenarios.

-   -   1) A CLS is received from the source port—the CLS is replicated        by the source port and sent to both the third port and the        destination port. The third port transmits the CLS to the third        node which handshakes with its own CLS which is returned to the        source port. The source port deletes the CLS from the third node        but remembers that it has arrived and releases the back channel        connection. The CLS reaches the destination port and the        destination node. The destination node handshakes by emitting        its own CLS which is transmitted back to the source port and to        the source node which closes. The source port drops the front        channel connection.    -   2) A CLS is received from the third node—the source port deletes        the CLS so that it does not get forwarded to the source node and        sends a CLS generated in the source port back to the third port,        and then drops the back channel connection to the third port.        The third port forwards the CLS to the third node. Any new third        port can then initiate a new dual simplex connection.    -   3) Destination node transmits a CLS—the CLS is received at the        source port and held by the source port. A copy of the CLS is        transmitted to the third port which transmits it to the third        node. The third node closes under normal FCAL close protocol and        handshakes by transmitting a CLS to the source port. The source        port transmits the CLS to the source node which closes under        normal FCAL close protocol rules and handshakes by transmitting        a CLS to the source port. That CLS is forwarded by the source        port to the destination port which forwards it to the        destination node. The source port then drops both the front and        back channel connections simultaneously.    -   4) Destination node transmits a CLS—the CLS is received at the        source port and held by the source port. A copy of the CLS is        transmitted to the third port which transmits it to the third        node. The source node closes under normal FCAL close protocol        and handshakes by transmitting a CLS to the source port. The        source port holds the CLS from the source node until a CLS is        received from the third node. Upon receipt of a CLS from the        third node, a CLS generated in the source port is simultaneously        transmitted to the destination port and the source node. The        source port then drops both the front and back channel        connections simultaneously.        Specific Dual Simplex Example

As a specific example of dual simplex in the context of FIG. 5 using ascoreboard which indicates dual simplex capability, refer to FIGS. 16Ato 16D which are a flowchart of the processing carried out by the portsto implement dual simplex communication. In this example, suppose node138 want to send data to node 101. Suppose also that node 144 wants tosend data to node 138. Node 138 generates an OPN(101, 138), RRDY, RRDYand transmits these three primitives to port 126 (step 360). Port 126converts the OPN to a simplex OPN(101, 101) and stores the original OPNincluding both the source and destination addresses of the OPN andstores the two RRDYs (steps 362 and 364). Port 126 looks up destinationaddress 101 in the routing table and finds it is connected to port 128(step 366). The scoreboard table is consulted, and port 128 is found tobe available (step 368). Port 126 posts a message on the protocol bus121 requesting port 128 to initiate arbitration for its local loop andpick a backplane channel (step 368). Port 128 wins control of its loopand sends a reply message naming the backplane channel 1 for use (step370). Ports 126 and 128 send commands to their portions of thedistributed crossbar switches to connect to backplane channel 1 (step372). Port 126 forwards the OPN(101, 101) to port 128 as soon as node128 is available and it passes straight through port 128 without delayto node 101 (step 374). The two RRDY primitives of buffer credittransmitted by node 138 and intended for node 101 are stripped off thetransmission to node 101 by port 126 and maintained as a count in thesource port for award by regeneration and transmission to another nodethat wants to ship frames to node 138. Node 101 replies with an RRDY(step 376). This RRDY is transmitted back on backplane channel 1 to port126 where, as described below, it is mixed in with data frames from athird node (node 144) that is transmitting data frames in dual simplexmode to node 138 so as to keep the source node 138 outputting new framesto the destination node 101.

In the embodiment considered here, port 126 posts a message to theprotocol bus to update all scoreboards that it is busy but is availablefor dual simplex connections. This message can be based upon the facteither that the source node knows by watching traffic or comparingsource node addresses on its local FCAL net to the AL-PS value in theoriginal OPN, that it is connected to a source node of the front channeland is capable receiving dual simplex transmissions, or itsconfiguration bit indicates dual simplex connections are allowed. Thismessage on the protocol bus would would result in update of allscoreboards (step 380). Node 114 then arbitrates for and wins control ofits FCAL net and generates an OPN(138, 144). Port 124 receives this OPN,stores it (and converts it to half duplex in the preferred embodiment)and uses the destination node address 144 to search the routing table(step 384). This returns the ID of port 126. This ID is used by port 124to search its scoreboard table. Port 124 finds port 126 in its routingtable, finding from its scoreboard that port 126 is busy but dualsimplex capable (step 386) and posting a connection request on protocolbus 121 requesting a connection to source port 126 (step 388). Thiswould result in a connect response reply message from port 126 grantingthe request and naming backplane channel 2 for use (step 390). Bothports 124 and 126 would connect to channel 2 (step 392), port 124 wouldsend its OPN(138, 144) to port 126 (step 394) which would cause port 126to compare the destination address 138 to the source node address (step396), find a match, quash the OPN and forward one RRDY to port 124 viachannel 2 (step 400). The backchannel transaction would proceed untilbuffer credit ran out, or third node had no more data frames to send ora CLS was sent by either node 138 or 144 (step 402).

In the preferred embodiment, port 124 would simply post a connectionrequests which would automatically be granted by port 126 with a replymessage saying use backplane channel 2. Ports 124 and 126 would bothconnect to backplane channel 2, and port 124 would send its OPN(138,144) to port 126. Port 126 would compare the source address it latchedfrom the original OPN, which is 138, to the destination address of theOPN received from node 124, which is also 138, and find a match. Port126 would quash the OPN(138, 144) and transmit one RRDY back to port 124which would reach node 144 and result in transmission of one frame ofdata. That frame of data would pass through backplane channel 2, port126 and the back channel path and reach node 138. Port 126 would thensend the remaining RRDY to port 124 which would result in another frameof data being transmitted. Any further RRDYs transmitted by node 138would be latched or counted and sent to port 124 in the normal course offlow control, or if node 144 sent a CLS, the RRDYs stored by port 126would be saved for grant to another third node until such time as theoriginal loop tenancy was terminated with a CLS from either the sourcenode or destination node.

In alternative embodiments, the port 126 can determine whether it iscapable of dual simplex by a sending a message to port 124 asking forthe destination address from the OPN (138, 144) and then compare thisdestination address to the latched source address from the originalOPN(101, 138). If there is a match, sending a reply message saying, “Usebackplane channel X and send me your OPN” whereupon the transactionproceeds as defined above. If there is no match, generating a CLS andsending it to the third port or sending a message that causes the thirdport to generate a CLS and send it to the third node.

Returning to the main example, port 126 mixes RRDYs received from node101 on the front channel connection with data frames received from node144 and sends the combined data stream to the source node over the backchannel of the FCAL net coupled to the source port (step 404). Thiskeeps source node 138 outputting data frames to destination node 101.

There are three scenarios for closing the dual simplex data paths. Anexample of each will be discussed in the next three paragraphs.

Suppose source node 138 issues a CLS (step 406). In that case, sourceport 126 duplicates the CLS and forwards it to destination port 128 andthird port 124 simultaneously (step 408). The third node 144 replieswith its own CLS which is forwarded to port 126 (step 410). Port 126discards the CLS received from third node 144 but notes that it hasarrived and both ports 124 and 126 relinquish their back channelconnections through the backplane (step 410). When the CLS reachesdestination node 101, it replies with its own CLS back to source node138, and ports 126 and 128 relinquish their front channel connections(step 412).

Now suppose third node 144 issues a CLS (step 414). In this case, theCLS is transmitted to source port 126 which deletes the CLS (it is notsent to the source node 138—step 416), and ports 124 and 126 eachrelinquish their back channel connection through the backplane (step418). Any new third node can initiate a new dual simplex connection.

Finally, suppose destination node 101 issues a CLS (step 420). Sourceport 126 receives the CLS and holds it temporarily and does not send itto source node 138 (step 422). A copy of the CLS is sent to third port124 and the third node (step 424). The third node replies with its ownCLS which is received by the source port and sent to the source node(step 424). The source node replies with a CLS which is forwarded to thedestination node (step 426). The source port and the destination andthird ports then drop the front channel and back channel connections(step 428).

Dual simplex mode allows many different third nodes to deliver frames tothe source node during its “conversation” (loop tenancy) with theoriginal destination node. Dual simplex almost doubles throughput of theswitch.

The Process of the Preferred Embodiment

An example of normal, non dual simplex processing by the switchillustrated in FIG. 5 (or any of the alternative embodiments of non dualsimplex switch operation) is illustrated by the flowchart of FIGS. 6Athrough 6C. This example is only a high level illustration of one paththrough the states of the loop port state machine of a single switchport such as loop port state machine (LPSM) 218 in FIG. 7 and isillustrative of typical processing for loop-local or chip-local orremote transactions. The LPSM in each switch port is a complex statemachine which has many states and many transistions between states withthe transitions between states depending upon the logical states ofvarious input signals. Each loop port state machine is comprised of aplurality of individual component state machines which interact witheach other. Some state machines generate output signals which are inputsto other state machines. This means that a large number of possiblescenarios for the states of the state machine switch ports exist. Toexplain all these states and all the conditions for transitiontherebetween, would obscure the main ideas of the invention. To exactlyillustrate all possible states and transitions, there is includedherewith as FIGS. 17 through 28 (each comprised of multiple figures suchas FIG. 17A and FIG. 17B, which should be pieced together at the cutline) are individual state diagrams for all the state machines in asingle Loop Port State Machine (LPSM). Each circle represents one state.Each line from one state to another represents a transition from onestate to another. The labels in boxes on each line represent the Booleancondition that must exist between the signals identified in the box forthat transition to be made. Boolean logical operators are identified by& for an AND operation, a tilde preceding a signal name means NOT and avertical line between two signals means OR between those two signals.Each state and each signal is defined in the verilog code appendedhereto as Appendix A. The state diagrams coupled with the verilog codetogether comprise a complete and exact description of all states andtransitions between states for every possible processing scenarioperformed by the preferred embodiment of the switch. A description ofthe verilog hardware description language and how to use it to definethe functionality of an integrated circuit is given in the treatise,Palnitkar, “Verilog® HDL: A Guide to Digital Design and Synthesis” ISBN0-13-451675-3 (Prentice Hall 1996) which is hereby incorporated byreference.

Local Destination Process

As a first example, suppose node 138 wishes to send data to node 140.Node 138 arbitrates for control of the FCAL net 142 comprised of datapaths 142 A, B and C and nodes 138 and 140 (step 150). When control iswon, source node 138 sends an OPN primitive to port 126 with thedestination address of destination node 140 therein (step 152).Optionally, the source node follows the OPN with one or more RRDYprimitives, each representing one frame buffer of credit which thesource node has reserved for receiving frames of data from thedestination node (step 152). Port 126 latches the OPN and any RRDYprimitive following the OPN (step 154), and uses the destination addressof the OPN as a search key to search a routing table stored in port 126(step 156). In the preferred embodiment, a separate routing table ismaintained in each port circuit. In alternative embodiments such asrepresented by FIG. 4, the routing table may be a single table 127coupled to all the ports by the protocol bus 121 with each port checkingthe routing table contents by messages on the protocol bus. Thisembodiment is represented by FIG. 4. In the embodiment of FIG. 4, thecontents of the routing table are updated by an active discovery processalthough this active discovery process may also be carried out in thepreferred embodiment of FIG. 5.

The contents of the routing table are destination addresses and theidentification codes (hereafter IDs) of the ports or half bridges towhich those destination nodes are coupled. Each half bridge containssuch a routing table. The contents of the routing table are learned byeach half bridge by watching the traffic on its ports in the mannerdescribed in the parent case or by an active discovery process whichwill be described below.

In this particular case, the search indicates that the destination nodeis local to FCAL net 142, and that determination is represented by test158 in FIG. 6A. Port 126 responds by setting an internal switch toconnect an internal bypass data path to connect data path 142C to datapath 142A (step 160), and sends the OPN to destination node 140 alongwith any RRDYs received from the source node (step 162). Node 140responds either with one or more RRDY primitives if it has buffercapacity to receive one or more frames from the source node or a CLSprimitive or one or more frames of data to be transmitted to the sourcenode in accordance with the number of RRDY primitives received from thesource node (step 164). In general, step 164 represents the process ofthe switch port acting as a normal non participating FCAL node for theduration of the transaction by forwarding all data and primitivesreceived from the local loop back onto the local loop. Thus, if aprimitive or data frame is output by the destination node fortransmission on data path 142B to source node 138 and node 138 respondswith an RRDY or data frame on path 142C, switch port 126 forwards theRRDY or data frame to the destination node via path 142A. If thedestination node outputs an RRDY, source node 138 responds bytransmitting an entire frame to node 140 via data paths 142C and 142Aand the internal bypass data path (not shown) within half bridge switchport 126 (or a number of frames equal to the number of RRDY primitivesreceived from the destination node may be transmitted—step 164). Theframe is stored in node 140, and when it has been processed and thebuffer is ready to receive another frame, another RRDY is output. If thedestination node responded with a frame of data in response to an RRDYreceived from the source node, the source node stores it in its buffer.If the destination node responded with a CLS primitive, the source noderelinquishes control of its FCAL net, and the transaction is endedaccording to normal FCAL close protocol. The process of exchanging RRDYsand frames of data continues until either node 140 outputs a CLSindicating it will send no more data to the source node or source node138 outputs a CLS indicating it will send no more data to thedestination node. The FCAL net 140 is then relinquished and other nodesincluding port 126 can arbitrate for control thereof.

Test 166 is symbolic of one event that can occur after the loop-localtransaction is completed. The loop-local transaction has to be completedsince no other node can win control of the local loop in arbitration soas to be able to send an OPN until the loop local transaction is over.Test 166 determines if an OPN has been received from the local loop,and, if so, vectors processing to step 154 to latch the OPN, assymbolized by step 168.

Test 170 symbolizes a test for an event which can occur regardless ofwhether the loop-local transaction is over or not—receiving a connectrequest message on the protocol bus at the switch port involved in theloop-local transaction. If this happens, the switch port becomes thedestination port for a remote transaction. In such a case, the switchport behaves as indicated in step 175 and the following steps toarbitrate for control of the local loop, as symbolized by block 172.When control is won (after the loop-local transaction is over), theswitch port sends back a connect response message naming the backplanechannel to use. The connection is then established by the source portand the destination port and the transaction is completed as indicatedin the steps following step 175.

If tests 166 and 170 do not detect either a new OPN from the local loopor a connect request on the protocol bus, the loop port state machineloops back to test 166 and stays in the state represented by tests 166and 170 until one of the events detailed there happens.

Remote Destination Process

Now suppose node 140 has data to send to node 144 coupled to port 124.Node 140 arbitrates for FCAL net 142, and when control is won, outputsan OPN with the destination address of node 144 therein (steps 150 and152, FIG. 6A). The source node may also output one or more RRDYprimitives. The OPN and any RRDY primitives are latched in the localnode (step 154), and the local node uses the destination address in theOPN to search the routing table (step 156). In this case, search of therouting table indicates that node 144 is coupled to a remote port 124,and an ID for the remote port is returned by the search (step 158). Ifthe search indicated the destination node was on an FCAL net coupled toa switch port on the same chip as the switch port coupled to the sourcenode, the result would be the same in that processing would be vectoredin either event to step 167.

Step 168 represents a determination as to the status of the remote portcoupled to the destination node. The combination of steps 168 and 176represent a determination of the status of the remote port and branchingto an appropriate routine depending upon the results. If the remote portstatus is “available” (path 170), source switch port 126 then sends aconnect request message formatted as in FIG. 14A via a protocol bus 121to the destination or chip-local switch port, as symbolized by step 172.In FIG. 14A, the requester ID is the ID of the source switch port andthe responder ID is the ID of the destination switch port. Although thisis the protocol of the preferred embodiment, in alternative embodiments,any protocol for determination of the status of the remote port and/orany other mechanism to establish a connection through the crossbarswitch or other connection mechanism will suffice to practice theinvention. In the preferred embodiment, the status determination isaccomplished by checking the status entry for the destination port in acopy of a scoreboard table stored by the switch port and kept up to datewith the copies of the scoreboard table in all other switch ports bymonitoring messages on the protocol bus. In alternative embodiments, thelocal port can send a message to the remote port via the protocol busasking it for its status or can send a message to a central scoreboardto get the status of the destination port.

If the remote port was busy, path 174 is taken from the statusdetermination test 176 back to step 168 to read the status again andprocessing stays in this loop in the local port until the status changesto available or no privilege. In some embodiments, a timeout functioncan be included to exit the busy-wait loop path 174 and send a CLS tothe source node after a timeout event to keep head end blocking frombarring all communications will all nodes coupled to the local port incase the remote node is broken and that would be equivalent to theclaimed invention.

If test 176 determines there is no privilege for the source node to talkto the destination node, path 178 is taken to block 180 where the localport sends a CLS to the source node. Processing then returns to Start.

Assuming the remote port was available and the connection request wasreceived, the remote port then starts arbitrating for control of itslocal FCAL net (step 175, FIG. 6B). When control is won, the remote portsends back a response message informing the first port coupled to thesource node to forward the OPN and any RRDY primitives it has latched,and telling the first port which backplane channel to use (step 177).

As symbolized by block 179, the response message from the remote portcauses both the first port and the remote port to generate commands tothe distributed crossbar switch circuitry of each port to set switchesto couple the two ports to the selected backplane channel to set up anFCAL loop connection between the first port and remote port through thecrossbar switch. In the example at bar, this data path couples port 126and port 124 through the backplane data path 130 on the channel named inthe reply message (step 179).

In some alternative embodiments, instead of sending a connection requestto the remote port, the first port will simply establish a data paththrough the backplane and send the latched OPN and any following RRDYsto the remote port 124. In response, port 124 will then latch the OPNand any following RRDY primitives, arbitrate for control of the FCAL netcoupled to node 144, and when control is obtained, transmit the OPN andany following RRDYs to destination node 144. If the local FCAL net isbusy, the remote port will hold the OPN and any following RRDYs untilthe FCAL net is available, and repetitively arbitrate for control andthen, when control is achieved, forward the OPN and RRDYs to thedestination node.

In this alternative embodiment, the destination node will respond to theOPN (and any following RRDYs) with either an RRDY if it has enoughbuffer space to receive a data frame or a CLS if it does not or with oneor more data frames to be transmitted to the source node correspondingto the number of RRDY primitives received from the source node if anyRRDY primitives were received. The transaction then proceeds bytransmitting one frame for every RRDY received from the other node untilone or the other of the source or destination nodes transmits a CLSprimitive. The frame or frames stream through ports 126 and 124 andbackplane data path 130 without ever being stored in any buffer memoryand without ever being segmented into packets that require segmentationand reassembly.

In the preferred embodiment, port 126 instead of sending the OPN and anyRRDYs to port 124 to signal it to establish a connection, instead sendsa connection request message via the protocol bus 121. The connectionrequest message tells the remote port 124 to arbitrate for its localloop and to notify port 126 when control is won and expect furthertraffic. When the response message is received that control has been wonby port 124 of its local loop and naming a channel to use on thebackplane for data transfer, the OPN and any RRDY primitives which werereceived by the first port from the source node are sent to port 124which then forwards these primitives to node 144 (step 181). As was thecase for the alternative embodiment discussed in the paragraph above,the destination node will respond with either an RRDY if it has enoughbuffer space to receive a data frame or a CLS if it does not or with oneor more data frames to be transmitted to the source node correspondingto the number of RRDY primitives received from the source node if anyRRDY primitives were received. The transaction then proceeds bytransmitting one frame for every RRDY received from the other node untilone or the other of the source or destination nodes transmits a CLSprimitive to complete the transaction.

The connection is closed with a handshaking type protocol that uses aswitch control character to eliminate race conditions. The connectionthrough the backplane can be closed by either the source node or thedestination node sending a CLS. Test 182 represents branching todifferent protocols for closing depending upon whether the source nodeor destination node initiated the CLS. Processing branches to step 183is the source node initiated the CLS. In step 183, the source nodeoutputs a CLS and that CLS is received by the source port and forwardedto the destination port over the backplane connection. Optionally, thesource port also sends an ARB(F7) or other innocuous fill word to thedestination port. The purpose of this fill word is to send affirmativecharacters to the destination port to prevent any spurious charactersfrom being transmitted to the destination port.

Step 185 represents the process of receiving the CLS at the destinationport and forwarding it onto the local loop so that the destination nodereceives it. The destination node responds with its own CLS.

Step 187 represents the process of receiving the destination node's CLSat the destination port and forwarding it across the backplaneconnection to the source port followed by a switch control character(typically 20 consecutive zeroes).

Step 189 represents the process of receiving the destination node's CLSat the source port and forwarding it onto the local loop so that it willbe received by the source node. The source port also loops the switchcontrol character back through the backplane connection to thedestination port.

Step 191 represents the process carried out in the destination port ofreceiving the switch control character transmitted by the source portand realizing the backplane channel in use is now clear. The destinationport then drops the backplane connection and transmits a release frameon the protocol bus. All other ports see that release frame and updatetheir scoreboards to indicate availability for that backplane channel.The switch ports involved in this transaction are now ready for a newtransaction so processing returns to start state 148.

In the case where the destination node initiates the CLS, test 182vectors processing to the closing sequence which starts with step 195.Step 195 represents the process of the destination node initiating aCLS. The CLS is received by the destination port and forwarded acrossthe backplane connection to the source port followed by a switch controlcharacter (currently 20 consecutive zero bits).

Step 197 represents the process that happens at the source port when theCLS and switch control character is received by the source port. Thesource port forwards the CLS onto the local loop and holds the switchcontrol character. The CLS reaches the source node, and it responds withits own CLS. This CLS is received by the source port and forwarded tothe destination port across the backplane connection. After forwardingthe CLS onto the backplane connection, the switch control character issent across the backplane connection. The source port then closes itsbackplane connection.

Step 199 represents the process which occurs at the destination portwhen the CLS and switch control character are received there. The CLS isforwarded onto the local loop by the destination port. There it isreceived by the destination node which causes the destination node toclose. The destination port then notes that it has received back theswitch control character it originally sent to the source port andcloses its backplane connection. The transaction is then over and thetwo switch ports are ready for a new transaction. Processing thusreturns to a start state represented by block 148 as symbolized by step193

Note in all of the above embodiments, the frame or frames stream throughports 126 and 124 and backplane data path 130 without ever being storedin any buffer memory and without ever being segmented into packets thatrequire segmentation and reassembly.

The lookup table in each port circuit is slightly different than thelookup table in the bridge described in the parent described in theparent application. There, only two half bridges were coupled together,so the routing table could output only a 1 if the destination was localor a 0 if it was not, or vice versa. With the many half bridges of theswitch, the routing table must store more bits for each destinationaddress, those bits (6 ID bits plus one bit for validity of the port)define the unique ID of the particular port to which each node having aparticular destination address is coupled.

The Scoreboard Table

In the preferred embodiment, a scoreboard table is used to store portstatus and priviledge information. Whenever the local FCAL net of a portis busy, the port connected to that FCAL net sends a message to updateits status in the scoreboard to “busy”. When the FCAL net becomesavailable again, the port connected to that net sends another message tothe scoreboard to update its status to “available”. The scoreboard canalso store privilege information to implement security rules to preventcertain ports from communicating with other ports. The status states mapto the following actions by the switching circuit: “busy camp” equalswait and do not forward OPN so no RRDYs get back to source node and nodata is sent; “available” equals connect and send OPN to destination;and “busy no camp” equals generate a CLS and send it back to the sourcenode, and “no privilege” equals generate an OPN and send back—this willtell the source node that the destinat node is not available.

Summary of the Preferred Routing Algorithm Using a Scoreboard andProtocol Bus

Thus, in the local and remote destination methods described above, thepreferred embodiment includes the following steps for consulting thescoreboard (this method is reflected in FIGS. 6A and 6B).

After the OPN and any following RRDYs from the source node are latched,the port that latched the OPN determines the port ID of the port coupledto the destination node by consulting the routing table as describedabove. After determining the ID of the remote destination port, thatport ID is used as a search key to consult the scoreboard table todetermine if the port is available and if there is the privilege to talkto it. If the remote ports status is “busy camp” and camping is allowed,send a connect request message and wait for the remote port to finishits current conversation, grant the connect request and send back aconnect response message naming the backplane channel to use. If theremote port's status is “no privilege”, generate an OPN and send it backto the source node. If the remote port's status is “available”, send aconnect request message to the remote port over the protocol bus. If thestatus if “busy no camp”, return a CLS to the source node. This causesthe remote port to arbitrate for and win control of its local loop. Whencontrol is won, the remote port then sends a reply message to update itsscoreboard status to busy and naming the backplane channel to use. Thescoreboard circuitry sees the reply message and updates the status ofthe port whose ID is in the reply message to busy. The reply messagecauses the first port to generate signals to the crossbar switch to opena connection between the two ports. The first port then sends thelatched OPN and any latched RRDYs to the second remote port coupled tothe destination node, and the process proceeds as described above untilthe transaction is completed. When the transaction is completed, bothports relinquish control of their FCAL nets and the destination portposts a message on the protocol bus that both the source and destinationports are availble. The scoreboard circuitry sees this messages andautomatically updates the status of each of these ports to “available”.

When search of the routing table with the destination address of the OPNreceived at the first port from the source node indicates that thedestination node is local, the scoreboard is not consulted and thebypass data path is activated. Then a message is sent by the first portto the scoreboard updating its status to “busy” to let all other portsknow that the FCAL net coupled to the first port is temporarilyunavailable for any other loop tenancy. Of course camping is allowed onlocal connection in the case of a “busy camp” status.

The status data in the scoreboard table will indicate a port is busywhen the FCAL net coupled to the port is tied up in a loop tenancy.

Referring to FIG. 7, there is shown a block diagram of a typical switchchip such as those represented by blocks 124, 126 and 128 in FIG. 5.Three FCAL net ports, each having a transmit and receive terminal, areshown at 200, 202 and 204. Each port has an FCAL net interface comprisedof a 1 Gbit/sec serializer/deserializer shown at 206, 208 and 210. Thefunction of these SERDES interfaces is to transmit and receive serialdata, converting the serial data stream to and from 10-bit charactersused internally in the switch chip.

The received 10-bit data data is re-timed in an elastic buffer, shown at212, 214 and 216, producing a 10-bit data stream whose timing isidentical to the transmitted data.

A loop port state machine (LPSM) for each port, shown at 218, 220, 222,functions to process received FCAL data and generate commands toestablish the proper connection to either the local FCAL net or to adesignated channel through the backplane so as to facilitate coupling ofthe received data on whatever data path the state machine establishes.The state machine also functions to receive data from the crossbarswitch coming from a remote port and couple it onto the FCAL net localloop of that port. The state machine also functions to do the routingtable searches and scoreboard table searches to determine when and FCALtransaction from the local loop must connect to the switch fabric and topost transaction messages on the protocol bus such as connectionrequests, reply messages, scoreboard updates etc. The switch fabricrefers to the backplane data paths and switching circuitry that allowsconnections between different ports. The state machine also functions toarbitrate for control of the local FCAL net when a connection request isreceived from the switch fabric, and, when control is achieved, pick abackplane channel and generate the reply message. Another function ofthe state machines in every port in dual simplex mode is to store thebuffer credit RRDYs output by the source node and forward these RRDYs tothe third port, one RRDY at a time up to the buffer credit limit, whenan OPN for a dual simplex transmission is received from a third node.Basically, all the logic and intelligence of each hub to carry out thefunctions described herein for the preferred or alternative embodimentsresides in the states of the state machine. The particular design of thestate machine is not critical, and any state machine (or programmablemachine if it is fast enough such as a microprocessor) which can performthe functions defined herein will suffice to practice the invention. Thenovelty is not believed to be in the particular design of the statemachine but in the functions it performs in each port.

Since each switching circuit such as 124 in FIG. 5 contains three ports(only one port is shown in FIG. 5), each switching circuit contains aport multiplexer, shown at 224 in FIG. 7. The port multiplexerinterfaces the three state machines or LPSMs to the backplane datachannels 226. The state machines control the multiplexer 224 to keeplocal destination traffic for each port local but to couple trafficbetween the port and a remote port to a channel on the backplane. Theport multiplexer and the state machine jointly determine the nature ofeach new conversation (local or remote) by searching a routing tablestored in memory 228. Preferably, memory 228 is a lookup table.

The backplane 226 comprises the datapath and control logic required toreceive and transmit the backplane data between switching circuits aswell as multiplexing circuitry to shunt backplane data to and from thestate machines via the port multiplexer 224. Backplane connections anddata channel availability are tracked by a backplane protocol circuitwhich manages scoreboards in each port which have their datasynchronized via broadcasts over the protocol bus 121.

The state of each switch chip is monitored by a management circuit thatinterfaces to the system through 2-wire bus 232. JTAG circuit 234 isused for testing the switch chip.

The serializer/deserializer function (hereafter SERDES) of each port at206, 208 and 210 is provided by the GigaBlaze SerialLink™ megafunctioncircuitry which is commercially available from LSI logic or anyequivalent SERDES circuitry, the details of which are herebyincorporated by reference. The SERDES circuits accepts differential,8b/10b encoded serial data at the rate of 1 Gbit/sec, demultiplexes itinto aligned 10-bit characters and recovers a receive clock from thedata. At the same time, the SERDES multiplexes outgoing 10-bitcharacters into a 1 Gbit/sec differential transmit data stream using aself-contained 10× clock multiplier. Other currently available SERDEScircuits may be used also either at macros in layout of the switch portchips or as external parts.

The elastic buffers 212, 214 and 216 absorb differences between receiveand transmit data rates which can vary up to 100 ppm from the nominalrate of 1.0625 Gbits/sec. The elastic buffers re-time the incoming datastream so that its timing is compatible with the transmit data. This isachieved by storing received data in a FIFO as previously stored data isremoved at a rate defined by the transmit clock. If the receive andtransmit rates are not equivalent, the receive and transmit pointerswill eventually converge. To prevent this, logic senses an impendingpointer collision, and repeats or deletes data when the FC-AL fill worddata is present in the FIFO. A fill word is a Fibre Channel ordered setdefined to be insertable or deletable as elasticity requirementsdictate. The FIFO also contains circuitry to perform a smoothingfunction to repair interframe gaps that have been made too small by theFIFO by deletions from a small interframe gap so as to make it so smallthat it creates problems downstream. The repair of interframe gaps isdone by inserting fill words into small gaps and removing fill wordsfrom large gaps. Specifications for such a FIFO are provided in the FCALstandards documents which are incorporated by reference herein.

The state machine in each port is similar to the FC-AL specificationstate machine but different in that it is passive and never transmits orreceives frames itself. The state machines 218, 220 and 222 relay OPNsand frames to remote destinations as well as performing the otherfunctions defined above. When a state machine receives data from thebackplane, it arbitrates for control of its local FCAL net with highpriority according to arbitration rules defined in the FCAL standardsincorporated by reference herein. Once control is achieved, the switchport then relays the OPN and frames onto the local FCAL net.

The state machines also implement the dual simplex process describedabove. Dual simplex mode can be turned on or off by management commandsreceived on bus 232. When this mode is on, the state machines convertfull duplex OPNs to remote devices to half duplex OPNs to the samedestination address. This leaves the source port available to receive anincoming connection from a third port.

The state machines do not need to participate in loop initialization,but rather they optionally allow initialization to occur as it would ina prior art FCAL net configuration and then change the state of theswitch to allow the state machine to actively engage source anddestination nodes and control data flow therebetween. Before switchingfrom hub mode (the switch can be instructed to act as a hub instead of aswitch), the state machines also function to learn the mapping betweeneach destination node address and the port ID of the port to which thatdestination node is connected. This learning can be performed by eithera discovery process or OPN trapping, and will be discussed below. Themappings learned in either process are written by the state machine tothe routing table in memory 228.

The port multiplexer 224 selectively connects each port to each otherwithin the switching circuit or to the backplane 226. The portmultiplexer has three multiplexer blocks each of which is controlled bya portmux state machine. Local traffic connections are implemented byconnecting a transmit data path and a receive data path between the twostate machines in the port multiplexer 224. Remote connections areestablished by connecting the transmit data path and receive data pathbetween the state machine and the backplane. Each channel through theport multiplexer actually implements two transmit and receive datapaths. In full duplex operation, only one TX-RX pair is used to carryhalf or full duplex conversation. When a port is configured fordual-simplex operation, the outgoing conversation uses one TX-RX pair totransmit data to and receive flow control primitives from thedestination node while the other pair is used to receive data from andtransmit flow control to any third node in the event a third nodeconnects to the source port.

The port multiplexer 224 also contains a camplist FIFO for each of thethree ports in the switch chip. These FIFOs are used to record the IDsof remote ports that have made a priority request to the switch port butwhich have not been serviced yet so as to implement the fairness tokenhighest priority to these remote ports which have not yet been servicedand prevent any starvation of a port.

The backplane 226 is a group of parallel point-to-point data paths whichphysically comprises 14 2-bit wide, 531.25 Mbit/sec data paths and a265.625 MHz strobe signal imlemented using differential LVDS to drivethe receiving switch chip. The switch chips are connected such that thebackplanes form a ring of point-to-point connections. The strobe isgenerated by an integrated PLL 227 and is timed with outgoing data suchthat it can be used to latch the data at the destination switch chipwhich is the next chip in the ring. Each data channel on the backplanemay operate in the pass-through mode, or it may be configured to alignincoming data into 20-bit words and steer data to and from a portmultiplexer channel, which typically happens when a remote connection isbeing serviced by a port. Each port can be configured to staticallylisten and transmit to certain channels to support broadcast.

The backplane logic tracks the availability of each backplane channel,and can be configured to associate a data channel with a receiving portand only release it when no other channels are available. This is usefulwhere backplane channels are underutilized to reduce latency.

The protocol bus is an 18-bit wide data bus used for broadcastingconnection requests and response messages to each switch chip in theswitch. The bus is arbitrated by external logic that is asynchronouswith respect to the main switch chip logic and thus can operate at anyspeed up to the limit determined by the system design. Each switch chipmonitors the protocol bus and updates the status information in its copyof the scoreboard such that every port knows the busy/available statusof every other port. This allows denial of full duplex connections atthe source if the destination port is busy. Dual simplex is an exceptionto this rule. The scoreboard also prevents loops of camped devices bydenying any camping connections to a port that is already camped onanother port.

The routing table in LUT memory 228 stores 6-bit mapping data betweeneach destination address and its port ID. Each port in a switch chipmust have received a unique ID assignment from external managementsoftware before the system can operate as a switch. The LUT memory hasthree 10-bit read address ports, one dedicated to each port on a switchchip. The LUT has one 10-bit write address port for writing associationsfrom the state machines developed in the learning process and also has7-bit read and write address ports used by management software whichdoes not form part of the invention. Internal logic in the LUT convertsthe 10-bit AL-PA destination addresses to 7-bit addresses for the 128×8SRAM. The output from the LUT is a 6-bit port ID that maps to the AL-PAinput at the address port of the lookup table, a valid entry bit and abit that indicates whether the destination node is local to the switchchip. When a port receives an OPN from its local loop, it uses LUT 228to determined if the conversation will be loop-local (destination onlocal loop), chip-local (destination on the local loop of another porton the same chip) or remote.

The LUT is initially loaded by a learning process called OPN trapping inwhich each port observes OPNs on its outbound local loop port and writesa destination address to LUT if a response to the outbound OPN on thelocal loop is detected on the local loop inbound data path. The portthen posts a message on the protocol bus that its local loop has thedestination address of the outbound OPN and giving its switch port ID.All the other ports receive this message and write the mapping intotheir routing tables.

The contents of the LUT are cleared by a state machine and countertriggered by a managment command or the rising edge of a chip resetsignal. Clearing the LUT takes 128 clock cycles.

Operational Overview

The switch chips of FIG. 7 can be operated in three fundamentalconfigurations: hub, switch and locked. Initial operation is in hub modewhere each port is coupled to its neighbor ports to form one big FibreChannel Arbitrated Loop (FC-AL). In locked mode, each port is standingby in preparation for a mode switch or broadcast. In switch mode, eachport forwards data back onto its local loop until an OPN to a remotenode is detected, and then a connection to the port coupled to theremote node is initiated through the backplane.

FIG. 8 illustrates hub mode. In hub mode, the state machines of allports cooperate to forward loop data from the receiver of one port tothe transmitter of the adjacent port through the port multiplexer 224.Because each switch chip can specify to the management software througha management-programmable register which hub mode backplane channel ituses, it is possible to partition the switch into several distinct loopsinstead of one big one. The hub channel is selectable on a per portbasis.

Switch mode the state machine of each port monitors traffic on its localloop and forwards data from its receiver to its transmitter when an OPNdesignating a remote node is received on the local loop. When thishappens, the state machine breaks the local loop and substitutes fillwords on the local loop transmitter so that nodes on the local loop donot know the loop has been broken. The OPN is held while a connectionattempt to the remote port is made. If the connection can be made, theremote port conducts unfair arbitration to win control of the remoteloop, breaks that loop if it wins control and transmit and receiveconnections between the two loops are initiated to form a loop acrossthe backplane between the source and destination nodes. FIG. 9 shows atypical switch connection between two ports on different chips as datapath 250, a simultaneous loop-local conversation represented by datapath 252 and a simultaneous chip-local conversation as data path 254.

At any time, a number of these connections may exist with the maximumnumber of chip to chip data paths limited by the number of backplanechannels.

Transactions that are purely loop-local proceed without intervention ofthe state machine in the preferred embodiment with the exception thatthe state machine momentarily holds the OPN received while it checks thedestination address in the LUT 228. If the destination is on the localloop, then the OPN is forwarded onto the local loop and the statemachine transitions into the busy monitoring state.

The state machine also detects a local tenancy by monitoring the RRDYprimitive, and the tenancy is terminated when a CLS is detected. At thetime a CLS is detected, the state machine returns to either a monitoringstate or to a remote arbitration state if a pending remote requestexists in the camp on FIFO (not shown in FIG. 7).

Chip-local transactions are handled by the port multiplexer 224 byarbitrating for local resources and physically multiplexing the data forall chip-local conversations. The state machine signals the portmultiplexer and forwards the destination address after receiving an OPNfrom the local loop naming a destination on the same chip but adifferent FCAL net. A hit on one of the other two ports in the chipinitiates the transmit request sequence.

The transmit requests to chip-local ports must be broadcast on theprotocol bus to resolve port contentions and deadlock issues. Localrequests proceed normally without posting to the protocol bus as nodeadlock is possible for local only transactions.

When a remote destination address is detected in an OPN from a localloop (done by the port multiplexer in some embodiments), the portmultiplexer forwards the request to the protocol bus logic 121 whicharbitrates for the bus and assembles a request frame. The destination orremote port receives the frame and either begins unfair arbitration forthe remote loop or queues the request in a FIFO-ordered list of campers.Eventually the request is serviced, and arb on the remote loop is won.At this time, the remote port issues a response frame on the protocolbus containing both grant and backplane channel ID on which to converse.The requesting port releases the OPN once the grant is received andawaits the first valid K28.5 primitive from the chose data channel. Theremote port, upon receiving the OPN primitive, releases its localtraffic onto the data channel and enters the connected state. The remotetenancy is considered terminated when the state machine has detected aCLS from both the source node and the destination node.

Each state machine implements a port bypass circuit and a parallelloopback mode, and each SERDES implements a serial loopback mode. Theport bypass and parallel loopback modes are also used to bypass a faultylocal loop while the switch is operating.

Port Bypass, Parallel Loopback and Serial Loopback

Each switch chip has three multiplexers that may be used to control theflow of data without regard to the FCAL protocol as illustrated in FIG.10. These three multiplexers control port bypass mode, parallel loopbackmode and serial loopback mode.

The port bypass circuit 260, when enabled, connects line 262 from theport multiplexer 224 in FIG. 7 back to the port multiplexer via line 264and port bypass switch. This has the effect of forwarding data from theport multiplexer 224 back to its source without travelling across thelocal loop coupled to transmit interface circuit 266 and receiveinterface circuit 268 coupled to the inbound and outbound data paths ofthe local FCAL net. SerDes (serializer/deserializer) circuit 270 doesthe serialization and deserialization function described above. Dataincoming from the local FCAL net is never sent to the port multiplexer224 when port bypass mode is engaged. The port bypass circuit 260 may beengaged by the managment software, and is automatically activated undercertain error conditions. It may only be disengaged by management. Innormal operation, 10-bit words generated by Giga Blaze circuit 271 onoutput 270 are coupled through port bypass switch 260 to the portmultiplexer via line 264.

Parallel loopback mode is manually engaged and disengaged by themanagement software, and is qualified by the port bypass enable. Thatis, parallel loopback mode is always disabled when port bypass isdisabled. When a switch chip is in parallel loopback mode, incominglocal loop data received by Giga Blaze circuit 271 at input 278 from thereceive interface circuit and output on line 270 is sent immediately tothe transmit interface circuit 266 via line 272, parallel loopbackswitch 274, Giga Blaze circuit 271 and output 276. This happens underall circumstances when parallel loopback mode is engaged. Data from theport multiplexer 224 is never sent to the local loop while parallelloopback mode is engaged. If port bypass is enabled an parallel loopbackis disabled, the loop is broken at the parallel loopback switch andARB(F7) primitives are transmitted onto the local loop.

Serial loopback mode is primarily used for testing the data paths in theswitch chip.

Priortity and Camping

All chip-local and remote connections fall into one of three priorityclasses: none, low and high. All connection requests from a switch portdefault to a static priority level (which is typically no priority)which can be set my management software. Priority for requests can beescalated to low or high by a number of mechanisms.

Priority escalates from none to low after a management programmablenumber of consecutive denials. If management software sets 0 as thenumber of denials, this escalation is disabled. After the trigger levelis exceeded, the priority deny count is reset and a low-priority requestbit is set for exactly one arbitration interval. This arbitrationinterval begins after the low priority trigger value has been exceededand an IDLE primitive has been detected on the inbound port of the statemachine. All subsequent remote requests will use low priority untilanother IDLE is detected, ending the arbitration interval.

The management software can also directly escalate priority on a port byenabling a rotating-token scheme in which each port requests at highpriority for a period of time while other ports deny requests to a port.

This mechanism guarantees each port a chance to service its localclients, at the expense of servicing any new remote requests. The tokenposition is computed independently by each switch chip through use of a6 bit token port ID location, a 6 bit token compare mask, a 16 bit tokenhold length register, and a single token priority enable bit. The tokenenable bit controls whether the token passing scheme is active in switchmode. If the enable bit is set to 1, the token passing scheme is enabledfor the switch chip. All switch chips must set their token enable bit tothe same value before transitioning to switch mode.

This mechanism allows for fairness to be maintained throughout thesystem. This feature is critical in congested server environments wherestarvation of disk access can freeze an application. The circulatingfairness token prevents this from happening.

The token position counter identifies the port or port group that holdsthe token. A port holds the token if the port ID of the port AND²-edwith the 6 bit compare mask matches the token counter value AND-ed withthe compare mask. If a port holds the token, its requests are made withhigh priority. Ports wishing to request a port that holds the token aredenied so that the camp list of the port holding the token can drain andlocal requests can be serviced.

The token position counter is incremented when a management-specifiednumber of wall clock counter bits roll over to zero. The wall clockcounter is a 24 bit counter which is incremented every word clock periodof 40 ns, resulting in a range of approximately ⅔ of a second. The tokenposition increment signal is computed OR-ing the lower 8 bits of thewall clock together with the bits resulting from an AND operationbetween the upper 16 bits of the wall clock and a 16 bit token lengthvalue, which should be set to a string of logic ones padded to the leftwith logic zero. When the resulting value is zero, the token counter isincremented.

The length of the string of logic ones in the token length registerdetermines the amount of time that each port will be assigned the token.For example, if the value of the token length register is 0 ¥ 3 f, thetoken counter will be incremented every time the lower 14 bits (8LSB's+6 additional bits) are zero, or every 2¹⁴*40 or about every 650usec.

Camping on a port means that a remote request waits on a busydestination port until that port becomes available. A non-camped requestwill simply be closed at the source if the destination port is busy.Campers are ordered in a camp list FIFO which has a depth of 8 entries.

Low and high priority are allowed to camp on a port if the camp list forthat port is not currently full and the desired port is not holding thetoken. High priority requests differ only in that at the destinationport they may be queued up to the depth of the camp list, while lowpriority requests queue to the low-priority camp depth set bymanagement. All requests are serviced by a single queue with FIFOdiscipline. An alternative embodiment uses separate queues for high andlow priority thus allowing high priority to jump to the head of theline.

Camping on a port that is already camping on another port can create adeadlock situation in which two or more camped ports form a cycle. Noforward progress can be made when this occurs, resulting in a “fatalembrace”. Such deadlocks are prevented by scoreboarding all ports whichhave outstanding requests. If the request has been observed on theprotocol bus but no response has been issued by the target port, thenthe requestor port is marked as camped in the scoreboard. All requeststo ports marked in the scoreboard as camped are immediately denied. Oncethe port in question has been issued a response, implying a grant and anallocated backplane channel, the formerly camped port can beginaccepting campers.

One anomaly exists which involves no-priority campers. Due toimplementation issues, it is easier to permit the very first request tocamp regardless of priority. Thus, even a no-priority request, if issuedto a port that is busy and has an empty set of camp lists, will becamped. This anomaly occurs because there are difficulties in detectinga busy port unless the port is involved in a remote conversation thatcan be reflected in the scoreboards.

Transitions from Hub to Switch

In order to transition the system from switch mode from hub mode, thesystem must first be locked. Before initiating a lock from hub mode, themanagement software should set all parameters necessary for switch modetransition while the system is in hub mode so that the time spent duringthe transition in the locked state is minimized. Since transactionscannot be initiated while the system is locked, it is possible that anNL_port could time out and LIP (initialize) if the system is locked fortoo long, causing the hub to switch transition to fail.

Once the system is locked, external logic asserts a signal which setsthe port multiplexers such that each port is partititioned from thesystem. During this time, each loop is broken, and IDLEs are driven ontothe local loops in order to nullify ARBs that may still be present fromhub mode arbitration. Next, the locking signal is deasserted, and, aftera brief delay in which IDLEs are sent to the outbound port, theswitching chips transition to the monitoring state and switch modeoperation commences.

If a LIP or other exceptional condition is detected during the switchmode transition, the system sets an output to a state that indicates themode transition failed and the system is still in hub mode.

Destination Node Address Discovery Processes

The associations between node addresses (AL_PAs) and their associatedports are determined at initialization time through a learning processreferred to herein as OPN trapping. An OPN primitive is said to betrapped in hub mode when it has been transmitted onto a local FCAL netand an RRDY primitive received in its place. When an OPN has beentrapped, the switch chip has the destination node address (AL_PD) of thedevice residing on the FCAL net where the trap occurred as well as theport ID of the port servicing the loop. This information is stored inthe LUT routing table 228.

Storing addresses into a single LUT is not sufficient for proper switchoperation. The data in the LUT routing table of each switch chip in thesystem should be filled in before transitioning to switch mode. This canbe achieved in two ways. The slower method, yet simpler to implement inthe circuitry of the system, is to use the management interface to readevery address in every LUT and build the LUT contents in software. TheCAMs of every switch chip are then loaded through the managementinterface.

The CAMs can also be synchronized as the OPNs are trapped throughcommunications on the protocol bus. Since the protocol bus is onlyutilized for scoreboarding during switch mode, every trapped OPN resultsin a protocol bus transaction that advertises the node address and theport ID to all switch chips.

The discovery of every utilized AL_PA through OPN trapping can befacilitated in a passive or an active manner. During hub mode, ifpassive OPN trapping is enabled, normal traffic will result in trappedOPNs which will eventually populate every LUT with the address-portassociations of every NL_port in the system.

AL_PA active discovery is a procedure by which the switch chip learningprocess is acelerated by additional logic on each switch chip. Activediscovery is initiated by asserting a signal on each switch chip whileit is in locked state. Once active discovery is initiated, a signalBCST_BUSY is driven low, and a 10-bit counter cycles through the 102410-bit words. Each word is passed through an address encoder whichgenerates a signal that is active whenever the 10-bit word correspondsto a legal AL_PA. For each such word, the discovery process is triggeredfor each port on the switch.

The active discovery process starts with an OPN transmitted onto thelocal loop using the counter word as the AL_PD or destination address.The OPN is immediately followed by an interframe gap and a CLS. Thissequence will travel around the loop and return to the discoverer if adevice with that particular AL_PD is not on the local loop. It the nodewith that destination address is on the local loop and the node is notbypassed, the node will consume the OPN and CLS and send a CLS of itsown. In this case, the discovery logic will write the AL_PA to portassociated into the CAM. After an OPN or CLS is received at the receiveport of the port which launched the OPN onto the local loop, the activediscovery state machine pauses until the counter reaches another validAL_PA. Once all valid addresses are attempted, the entire ArbitratedLoop address space has been deterministically mapped. This causes theBCST_BUSY signal to be allowed to float to logic 1, signalling thatactive discovery is complete. Active discovery may be aborted bydeasserting the signal which initiated it which results in immediatereset of the discovery state machine.

Loop Port State Machine Policy

The loop port state machine is not a strict implementation of aconventional FCAL loop port state machine since it must switch OPNsequences and subsequent frames to and from the backplane and postmessages to and read messages from the protocol bus and update thescoreboard and carry out the learning protocols.

While the exact design of the state machine is not critical to theinvention, all designs must be able to perform the functions describedabove for at least one embodiment and, at least in the preferredembodiment, must adhere the following rules.

-   1) The LPSM must preserve word alignment unless it is absolutely    impossible to do so. Since bugs exist in the HP Tachyon™ design    which fill the loop with nonsensical transmissions if a K28.5 is    followed by a character and another K28.5, any LPSM implementation    that shares the loop with a Tachyon-based device must exercise great    care in aligning output words.-   2) The LPSM must not disturb OPN-CLS or SOF-EOF symmetry, nor should    it unintentionally disturb credit flow by removing and adding RRDYs.    In dual simplex cases, the LPSM will intentionally manipulate credit    by absorbing RRDYs and rerouting them to an alternate destination.-   3) The LPSM must conform to the FCAL requirements regarding    interframe gaps. Also, the LPSM should avoid interframe gap    conditions which are FCAL legal, but cause interoperability problems    with current equipment.-   4) The LPSM may operate in unfair mode in order to prioritize remote    connections over local connections, but it must not break    arbitration by prematurely resetting the access window, or    starvation may occur.-   5) The LPSM must allow loop initialization to be triggered and to    proceed in a reasonably normal manner.

FIG. 11 is state table describing the various LPSM states in thepreferred embodiment.

Loop Port Unfairness

An LPSM always operates in unfair access mode in order to obtain theloop as soon as possible. The current arbitration window is not observedand the high priority ARB (usually ARB(0), but programmable by themanagement software to be any AL_PA) is used to gain control of theloop. Simply put, the loop port state machine issues ARB on the loopwhenever a remote request is pending and continues to do so until thesame ARB is received back at the LPSM indicating that the LPSM has wonarbitration.

If the LPSM receives an OPN or RRDY primitive, then another device onthe loop won arbitration. In this case, the LPSM continues to issue itsARB, and monitors the loop until arbitration is won. While arbitrating,an OPN may arrive from the loop; the AL_PD of the OPN is then used tosearch the LUT to determine whether the OPN is to a remote or localdevice. If the OPN is remote, the LPSM synthesizes a CLS in an attemptto end the tenancy of the device on the local loop which sent the OPNwhich is interfering with the arbitration attempt of the LPSM. Thus,remote tenancies which have already traversed the protocol bus and arepresent camped are heavily favored over local devices who are attemptingto make a remote connection. If however the OPN received from the localloop during arbitration by the LPSM is directed to a local device, theLPSM continues to arbitrate while processing the local transactionnormally.

Interframe Gaps

The ports on every switch actively participate on their loops when theyare facilitating a remote tenancy. The two primitives RRDY and CLS areused according to FCAL net interframe gap policy. The specific policythat the switch chips use is that if a current primitive is a fill wordand two consecutive fill words have previously been detected, theninsertion of the RRDY or CLS may take place. For purposes of thispolicy, fill words are defined as ARB(x) and IDLE primitives.

Fill Word Insertion

Fill words are generated and inserted onto the local loop when the LPSMis not in the LPSM_MON state (hereafter the states of the LPSM will bereferred to by the acronym that follows LPSM_ in FIG. 11). Rulesgoverning fill word generation ensure that the current arbitration stateof each loop is maintained independently. The problem for each switchchip when it is coupled to a source loop, i.e., the loop having thesource node, is that it must send fill words that are relativelyinnocuous. An NL_port in an OPEN state will transmit ARB(F0) which ituses to determine if any other ports on the loop are currentlyparticipating in arbitration. If the open NL_port receives ARB(F0) inreturn, the NL_port has the option of retaining the loop and sets itcurrent fill word to IDLE, thereby resetting the arbitration interval.The LPSM needs to send a management programmable low priority ARB thatdoes not possess the potential to disturb the local loop access statelike ARB(F0) does. For this purpose, the ARB(F7) is suitable, as it isonly used by a non-participating port to quiesce the loop prior tosending a loop initialization primitive LIP in order to receive a validAL_PA.

The fill word generation matrix for the source loop is given in FIG. 12,and the destination fill word generation matrix is given in FIG. 13. Inthe source loop LPSM, the ARB(F7) state is entered when a local OPN isreceived, and a minimum of 6 current fill word primitives are sent whenrelinquishing the loop. In the destinaton loop LPSM, the ARB(F0) stateis entered when a remote connection request is received, and the ARB(F0)state is entered when a tenancy is established and an OPN is passed. Aminimum of 6 current fill words are sent when relinquishing the loop.

Remote Data Transfer

In order to reduce the amount of decoding required to merge the datastream of a remote connection into the flow of data on the local loop,very few primitives are passed across the backplane.

Miscellaneous Primitive Handling

The most common primitives that the switch chips must deal with are:IDLE, RRDY, ARB, OPN, SOF, EOF, CLS and LIP. The other primitives thatmay be encountered include: NOS, OLS, LR, LRR, MRK, LPB AND DHD. Thehandling of these other primitives is described below.

NOS, OLS, LR and LRR Primitives

When NOS or OLS primitives are detected, on the inbound port of a loop,the OLD_PORT detect flag of the port is set so that the managementsoftware can detect the condition. LR and LRR primitives will be fedback onto the local loop, but not through the backplane.

MRK Primitive

The MRK primitive is discarded if issued during a remote tenancy indual-simplex. Otherwise, MRK primitives are passed around the local loopand across any remote connection. However, it is unlikely that a MRKwill find its destination unless it is issued inside of a tenancy inwhich the target is on the destination loop.

LPB and LPE Primitives

The LPB and LPE primitives are trapped and written to managementsoftware. A detection flag is set for either an LPB or an LPE, and theAL_PD and AL_PS are written to a 20-bit register that is accessible bythe management software.

DHD Primitives

In dual simplex mode, all dynamic half duplex primitives are translatedinto CLS primitives. Otherwise, they are passed unaltered to theirdestination.

Protocol Bus Definition

The protocol bus is the medium by which the scoreboards for each switchchip are kep current. It also serves to communicate switch connectionrequests and responses between switch ports. The protocol bus is definedas an 18 bit bidirectional data bus named PBD, a request output PBREQ0for each switch chip, a grant input named PGGRNT1 to each switch chip, ashared bus idle input signal PBIDLE1, and shared frame available inputsignals named PBFRM1. The protocol bus runs asynchronously with respectto the 106.25 MHz core circuitry to which it interfaces. It is onlynecessary to provide a clock of less than 50 MHz for the bus to functionproperly.

When a switch chip wishes to transmit a data frame on the protocol bus,it drives its PBREQ0 output. The PBREQ0 of each switch chip in thesystem is fed into external arbitration logic which prioritizes theinputs and asserts a single PBGRNT1 output in the same cycle. The chipthat receives the asserted PBGRNT1 deasserts PBREQ0 and then drives thePBD bus in the next cycle. The arbiter asserts PBFRM1 in the cyclefollowing any PBGRNT1 to notify each switch chip that a data frame ispresent on the protocol bus.

Once a switch chip obtains a PBGRNT1, it must not assert PBREQ0 until itsees PBIDLE1 asserted from the arbiter. The PBIDLE1 is asserted wheneverno chip is requesting the bus in a cycle, indicating that the protocolbus arbitration window may be reset.

Each protocol bus data frame possesses a slightly different format.These formats are depicted in FIGS. 14A through 14E. The abbreviationsBP CHNL in FIGS. 14B and 14C stand for backplane channel. Theabbreviation PID in FIG. 14E stands for port ID.

The connection request data frame of FIG. 14A is sent by a port thatwishes to initiate a connection to a remote destination port and hasfound that the destination camp list can accept a request of the desiredpriority, and that the destination port is not current requesting adestination for itself. When a connection request is broadcast, everyswitch chip marks its 64 bit requester scoreboard and increments itscamp list depth scoreboard for the destination and priority.

Chip remote request frames have a 0x0 pattern in the LCL field shown at300 in FIG. 14A while chip local requests place a 0x3 in this field.This difference in format allows switch chips to update channelallocation scoreboards only on chip remote requests. If a chip remoterequest hits a channel that has been placed in an age list, a channelidle scoreboard for the channel owned by the destination port iscleared; otherwise, a need counter is incremented as will be explainedin the next section.

A connection response having the format of FIG. 14B is broadcast by aport that has serviced a camp list entry, and indicates that aconnection may be initiated over a backplane data channel specified infield 302. A connection response results in the clearing of therequestor scoreboard for the requester ID given in field 304 of theresponse frame, and it also results in setting the responder scoreboardfor the destination port. The port that own the allocated channel andthe channel number are entered into the age list on the initialallocation of the channel. A responder port that resides on the sameswitch chip as the requestor, indicating a chip-local connection, muststill broadcast a response frame so that the responder scoreboard isupdated in all the other switch chips. This type of chip-local connectrespose carries an invalid channel ID code of 0xf, which indicates toreceiving switch chips that the backplane usage scoreboard should not bemodified.

A responder channel idle frame having the format of FIG. 14C is drivenon the protocol bus when a backplane channel completes a transaction. Inthe default system configuration, the channel is retained by theresponder until the channel is required by another responder, thusreducing setup overhead if the destination node that just finished atenancy over the channel is a frequently accessed destination. When anidle frame is received, each switch chip updates its channel idlescoreboard to indicate the channel is available.

The no-op frame of FIG. 14D is used as a late abort of a response due tochannel allocation. This may occur when a request has been queued fortransmission but a conflicting request is received before the queuedrequest can be transmitted. The queued request is checked a final timebefore transmission, and if a local deny must take place, the frame isconverted to a no-op and sent to get it out of the queue. Any other wayof purging the queue will also suffice.

The lookup table update frame of FIG. 14E is transmitted when an OPN istrapped on a local loop in hub or locked mode. The frame is then used byeach switch chip to enter a node address AL_PA equal to the content offield into its LUT with a port ID mapping as specified in field 308.

Backplane Channel Allocation

The example given herein for the configuration of the backplane has 14separate backplane data channels, three of which may be dedicated toother uses such as broadcast. Channels are allocated by destinationports and remain assigned to them for as long as possible. A channelscoreboard indicates if any channels are free and is used to hold offany pending responses from ports which are not already connected to adata channel. If this is not the case, a need counter is incremented.When this need counter exceeds the physical number of data channels,then the backplane attempts to free up a channel while the destinationport attempts to obtain a connection grant response from its LPSM. Alldata channel connections (identified by response frames on the protocolbus) are stored in the age list which indicates the channels that havebeen held the longest and the port that currently own those channels.When the need counter exceeds the number of data channels, each switchchip consults its age list and selects the channel to be relinquished.Idle channels are broadcast on the protocol bus when the camp list ofthe port owning the channel empties completely. The oldest owned channelis freed by the switch chips that own the channel and all switch chipsupdate their scoreboards to reflect the new state. For every channelthat is freed, the need count is decremented by one.

Channels can also be freed in blocks of size greater than one. The sizeof the block of channels freed is determined by management-programmableparameter. The switch chip management logic can also instruct thebackplane to always free a channel when it becomes idle, rather thanonly freeing the channel when the need arises.

Switch Fairness

Starvation is a problem because most of the fairness policiesimplemented by the switch chips guarantee remote access fairness toswitch ports and not to the individual NL_ports. Thus, while an NL_portis guaranteed access to its local loop and is guaranteed to winarbitration within one FCAL net access window, its tenancy will not besuccessful if the NL_port attempts to transmit to a remote port and isdenied connection by the switch. Usually, randomness of access patternswill result in statistical fairnesss. However, it is not uncommon toencounter degenerate cases where this behavior can occur repeatedly on agiven NL_port, resulting in starvation of the port.

To improve the fairness of access to the switch at the NL_portgranularity level, it is necessary to guarantee that all NL_ports on alocal loop get some fraction of the switch bandwidth. Mechanisms existfor escalating from no-priority requests to low-priority requests basedon the number of consecutive denials of access issued by a remote portstate machine and for rotating a fairness token among all the ports toguarantee that each port gets a turn at high priority access.

Broadcast Support

In the preferred embodiment, each switch chip's LPSM includes logic tosupport three possible broadcast modes: that of the broadcast sender,the broadcast server transceiver, and the broadcast receiver. For aswitch chip to be able to send broadcasts, it must include logic todecode a broadcast sequence as described below, logic to determinewhether the broadcast channel, if it exists, is busy, logic to forward abroadcast sequence directly to the local loop as it is received, andlogic to write a preamble to the broadcast sequence and place thesequence on the broadcast channel if it is available.

To decode a broadcast sequence, each switch chip should have the thefollowing structure. Broadcast sequences are detected on the inboundport from the local loop. A broadcast sequence is defined to be thesequence of one or more broadcast OPN groups terminated by a CLSprimitive. A broadcast OPN group is defined to be one or more selectiveor broadcast OPNs followed by data frames.

The LPSM always forwards a broadcast OPN group back to its local loop,guaranteeing that all local NL_ports see the broadcast and that thesender of the broadcast can clean up the local loop. If broadcast modeis enabled by the management software, the switch port also attempts toforward the broadcast to the predefined backplane broadcast channel. Ifthe channel is not already busy with a broadcast that is already inflight, the switch chip will prepend a unique, identifier sequence tothe broadcast sequence and transmit the prepended sequence onto thebroadcast channel. During the transmission, any broadcasts on thechannel but upstream from the broadcasting switch chip will bediscarded. As a result, only one broadcast may reach the broadcastserver. When the server receives a broadcast, it reads the broadcasterID prepended to the sequence. This identification mechanism excludes theoriginal broadcaster from transmitting data onto the local loop a secondtime. The broadcast server then sends unicast to each of thedestinations on the broadcast list.

The server comprises one port of a switch chip that is configured toonly listen and transmit on the broadcast channel, and a broadcastserver board that is connected to the switch chip by its serial loopport. Broadcast sequences are received by the dedicated broadcast portand forwarded to the switch port connected to the broadcast server boardfor storage.

In order for a switch chip to be able to receive broadcasts, the LPSMmust include logic that discriminates whether an incoming broadcastoriginated at the port or at some other port from the data in thepreamble prepended to the broadcast sequence.

In order for a switch chip to be able to function as the transceiverinterface to the broadcast server, a switch port must be configurable sothat it only forwards data from the broadcast channel to the loop portand forwards data from the loop port back to the broadcast channel. Thisport should not be effected by state change requests or otherexceptional conditions if it is configured as a broadcast transceiver.

Buffered FCAL Switch

Referring to FIG. 29 there is shown a species of a buffered FCAL switchwhich falls within a separate second genus of FCAL switches, suitablefor Class 3 Fibre Channel operation only. Switches in this genus stilluses the destination address in the OPN to find the remote port but usesbuffers instead of hold back flow control to complete the transaction tobusy remote ports. Specifically, species within this genus will use thedestination address of the OPN from the source node to find the locationof the remote port. The destination address is used to look up the portID of the switch port couped to the destination address. Suppose Thenthe status of that port will be checked. If the status is available, aconnection request will cause a connection to be set up between thesource node and the destination node via a source port connected to thesource node and a destination port connected to the destination node.The buffer comes into play when the destination port is busy. In thissituation, in the first genus described above, the normal primitives ofthe FCAL protocol are used for flow control to prevent the source nodefrom transmitting any frames of data until the destination port becomesavailable. In the second genus defined in this paragraph. a buffer bigenough to hold one or more complete frames of data is included in thefront end of each switch chip, or multiple buffers each big enough tostore a frame of data are included with each switch chip front end. Eachof these buffers will serve as an auxiliary switch port and have its ownconnection to the backplane in some species or a single sharedconnection to the backplane through a multiplexer can be used. Thepreferred species uses multiple buffers each with its own connection tothe backplane in addition to a connection directly from the switch portto the backplane for direct connections without buffering. In somespecies, a single shared buffer or multiple shared buffers on thebackplane or in some central location may be used.

In this second genus, the way the buffers are used is for the sourceport to generate an RRDY sua sponte when it finds from a check of thescoreboard that the destination port is busy. The RRDY is sent to thesource node and causes it to output a frame of data. This frame of datais stored in the switch port's buffer. Then a message is sent to thedestination port indicating that the auxiliary buffer of the switch portis holding a frame of data for the destination port. This auxiliarybuffer ID is added to the camp list for the destination port. When thedestination port becomes available, a message is sent back on theprotocol bus indicating that the destination port is now available andnaming the backplane channel to use. A connection through the backplaneis then established to this channel by the auxiliary buffer connectioncircuitry and the destination port, and the data in the auxiliary bufferis transmitted. If the switch port has multiple auxiliary buffers, theyeach have their own IDs and, preferably, each has its own switchingcircuitry to make a connection to the backplane.

In this second genus, each auxiliary buffer has circuitry coupled to thereturn path to recognize RRDYs transmitted back by the destination nodeand to count them (or store them) and to wait for a connection betweenthe source port and the RRDY counting circuit if the connection is notcontinuous such as in some cases where multiple buffers are present ineach switch port. These stored RRDYs (or self generated in the case of acount only) can be transmitted to the source node in the case of fullduplex or mixed with frames from a third node in the case of a dualsimplex connection and transmitted to the source node. Each source portalso has shared circuitry for each FCAL net which recognizes incomingRRDYs from the source node and counts them or stores them. These sourcenode generate RRDYs can be transmitted to the destination node in thecase of full duplex or transmitted to a third node in the case of dualsimplex.

The operation of such a switch is now described with more specificity inconnection with the species shown in FIG. 29. We first consider a fullduplex transaction. In the species of FIG. 29, an OPN/RRDY detector andRRDY generator circuit 450 detects any incoming OPN or RRDY primitivescoming in from the source node 452 on the local FCAL net 454. Thedestination address of the OPN is latched and sent to the lookup tablecircuitry represented by block 456 via line. The lookup table looks upthe ID of the port connected to the destination node and determines itsstatus. Suppose a first OPN with a destination address for node 451 isreceived from source node 452 and the status of destination node 451 andits switch port 453 is available. In this case, protocol bus interfacecircuitry in block 456 sends a connection request via protocol bus 458to protocol bus interface circuitry in block 457 of switch port 453requesting a connection. This results in a connect response messagenaming the backplane channel to use. The loop port state machines inblocks 456 and 457 then send commands via buses 466 and 467,respectively, to their respective port multiplexers 460 and 459 toestablish connections between “straight through” data paths 462 and 464to the designated backplane channel in backplane 465. The LPSM in block456 also controls switches 468 and 470 to make connections to wires 462and 464, and the LPSM in block 457 control switches 469 and 471 to makeconnections to “straight through lines 473 and 475. Destination noderesponds with an RRDY and that gets passed to source node 452 throughthe connection just described and the transaction proceeds normallyuntil completed and the backplane channel is relinquished.

Now suppose switch port 453 was busy. In this scenario, the lookupprocess using the destination address in the OPN determines that animmediate connection is not possible. In this case, the LPSM in block456 controls switch 470 to make an outbound connection to buffer 1A viawire 472 and controls switch 468 to make an inbound connection via wire474. The LPSM then commands switch 476 to open long enough for an RRDYto be sent to source node 452 and commands RRDY generator circuit 450 togenerate an RRDY on line 478 and send it to source node 452. The RRDYcauses source node 452 to output a frame of data. This frame passesthrough line 480, SERDES 482, switch 470 and line 472 into buffer 1A andis stored there. The circuit 450 then generates a CLS and sends it tosource node 452. Then switch 476 is closed. LPSM in block 456 then sendsa message on protocol bus 458 to the protocol bus interface circuitryand LPSM in block 457 that it has a frame of data waiting fordestination node 451. This message gives the ID for the buffer 1A andcauses that ID to be put on a camp list for destination node 451. Whendestination node 451 becomes available, LPSM in block 457 sends amessage back to LPSM in block 456 saying “send data in buffer 1A onbackplane channel X”. The LPSM in block 456 then controls portmultiplexer 460 to establish a connection between wires 484 and 486 tothe designated backplane channel. LPSM in block 458 controls switchs 469and 471 to establish connections to wires 473 and 475 and controls portmultiplexer 459 to connect wires 473 and 475 to the designated backplanechannel. The LPSM in block 456 then causes a switch 490 to close andcauses Buffer 1A to output its frame of data onto the backplane channelwhere it gets transmitted to the destination node 451 via straightthrough wires 473 and 475.

In some embodiments, the circuit 450 will not send a CLS to the sourcenode 452 after it sends a frame of data into the buffer 1A. In theseembodiments, the connections will be maintained so that if there is morethan one frame, it can be sent as soon as the destination node becomesfree by a straight through connection. In these embodiments, the LPSM inblock 456 also causes switch 492 to close for a return path and causesswitch 468 to make a connection to return path wire 474 so any RRDYsoutput by the destination node are transmitted to the source node 452once the destination node becomes available. The LPSM also then causesswitch 470 to make a connection between outbound path 480 from sourcenode 452 and “straight through” outbound wire 464 so subsequent framescan be sent straight through. The transaction then completes as astraight through transaction. Buffers in the switch ports such asbuffers 1A and 1B may be big enough to hold more than one frame in somespecies to avoid having to establish a separate tenancy for each frame.

Since the preferred method in the buffered switch is to close the sourcenode after it outputs one frame into the buffer, it is then free togenerate new OPNs to other destination nodes. Those OPNs can causeeither straight through or buffered connections to their destinationsdepending upon the status of the destination port. The LPSMs cancooperate after the table lookup using the new OPN destination addressto control the switches 468 and 470 and their counterparts in thedestination port to establish a straight through connection to the newdestination via another backplane channel. In the meantime, the buffer1A and its associated switches and the LPSM are cooperating to act as anindependent switch port such that when the destination node for the datain the buffer becomes available and a connect response message isreceived, it can independently send its data to the destination node thedifferent backplane channel assigned in the connect request message.This increases throughput since the same switch port may besimultaneously be sending data to two or more different destinations.Buffered FCAL switches such as that shown in FIG. 29 have the additionaladvantage in that it prevents source blocking. The source may downloadone or more frames into the buffer for a destination node and then moveon to its next transaction thereby reducing or eliminating head endblocking.

Dual simplex is also possible to increase throughput further by usingcircuit 450 to count RRDYs emitted by the source node and using LPSM inblock 456 to award those RRDYs to a third node. This way, the RRDYs willcause the third node to send data inbound to the source node via aseparate backchannel connection with LPSM in block 456 controllingswitch 468 to establish the inbound path to source node 452 via thebackplane channel assigned by the third node and straight through wire462. Outbound data from the source node to a destination node can besent by straight through wire 464 or through one of the buffers. RRDYsemitted by the destination node are received by the destination port andsent via the protocol bus to the LPSM of the source port or a message issent each time an RRDY is received from the destination node by thedestination port informing the source port of this fact. The source portLPSM then controls circuit 450 to generate an equivalent number of RRDYsand mix them in with the data frames from the third node on the inboundpath 492 to the source node 452 to keep the source node outputting dataframes destined for the destination node.

The RRDY capture circuits are used in species where RRDYs are emitted bythe destination node, but the switches 468 and 470 are in states suchthat the RRDYs cannot be immediately be sent to the destination nodesuch as where the source port is generating multiple OPNs to differentdestinations and filling up all its buffers one by one. In theseembodiments, the nodes will have to be non standard in that they willhave to OPN a destination, download one or more frames into a buffer,receive a CLS from circuit 450, OPN a new destination, download anotherone or more frames into another buffer, receive another CLS from circuit450 and continue this process until all buffers have been used. The LPSMwill commutate the switches to make successive connections to thebuffers and straight through connections as needed and keep cyclingthrough these connections. The RRDY capture circuits will count thenumber or RRDYs received or emitted by the destination nodes, and whenthe switches 468 and 470 and 490 and 492 are again in position forcommunication with the destination node, the source node will openitself spontaneously for that destination, receive any stored RRDYs fromthat destination and send an appropriate number of frames to thatdestination either by a straight through connection or a bufferedconnection.

Fairness is implemented in the species within the second genus in thesame way as in the species of the first genus. A fairness token iscirculated, and when any switch port has the token, it assumes thehighest priority. The fairness token can be circulated amond the switchports on a separate fairness token bus (not shown in FIG. 29) or via theprotocol bus which is the embodiment symbolized by FIG. 29.

Although not shown for simplicity in FIG. 29, each switch port alsoincludes a local loop bypass data path to keep purely local transactionsconfined to the local FCAL net. Note also that although each FCAL net isFIG. 29 is shown as having only one node, multiple nodes on each FCALnet are also possible. Note also that although FIG. 29 shows only oneswitch port per switch chip, multiple switch ports per switch chip arealso possible similar to the structure shown in FIG. 7 but using thearchitecture of FIG. 29 for each switch port.

Appendix B attached is the UUencoded Verilog description of thepreferred embodiment of the entire switch chip integrated circuit. Atthe end of Appendix B is the C language source code for the UUencodingand decoding program to enable decoding of the Verilog and documentationfor using the UUencoding and decoding program.

Although the invention has been described in terms of the preferred andalternative embodiments disclosed herein, those skilled in the art willappreciate numerous modifications that can be made. All suchmodifications and alternatives are intended to be included within thescope of the claims appended hereto.

1. A process for insuring fairness in an switch having a plurality ofswitch ports, each coupled to a fibre channel arbitrated loop (FCAL) nethaving one or more nodes coupled thereto, comprising: assigning eachswitch port a priority level for use in arbitration between conflictingconnect requests from multiple switch ports to an identified destinationport; circulating a fairness token among said switch ports so that eachswitch port receives said fairness token, holds it for a time and thenforwards it to another switch port; assigning a switch port that hassaid fairness token in its possession a highest one of said assignedpriority levels; and denying access to a switch port of the plurality ofswitch ports that possesses the fairness token until local requests forthe port can be serviced.
 2. A process for insuring fairness is achievedin an switch having a plurality of switch ports, each coupled to a anfibre channel arbitrated loop (FCAL) net having one or more nodescoupled thereto, comprising: assigning each switch port a priority levelfor use in arbitration between conflicting connect requests frommultiple switch ports to an identified destination port; circulating afairness token among said switch ports so that each switch port receivessaid fairness token, holds it for a time and then forwards it to anotherswitch port; assigning a switch port that has said fairness token in itspossession a highest one of said assigned priority levels; keeping acamp list for busy ports which have received multiple connect requestsfrom different switch ports; when a switch port in possession of saidfairness token makes a connect request to a switch port with a camp listwhich is not full, adding the ID of said switch port having saidfairness token to said camp list; when access to said switch port withthe camp list is granted to the switch port in possession of thefairness token, forwarding said fairness token to another switch port.3. The process of claim 1 further comprising: providing a token positioncounter that identifies a port that holds the token; and incrementingthe token position counter in a predetermined interval of time.
 4. Theprocess of claim 3, wherein the predetermined interval of time is basedon a token length register.
 5. An apparatus for insuring fairness in aswitch having a plurality of switch ports, each coupled to a fibrechannel arbitrated loop (FCAL) net having one or more nodes coupledthereto, the apparatus comprising: a circulating unit configured tocirculate a token between the plurality of switch ports; and a tokenposition counter configured to identify which of the plurality of switchports holds the token, wherein requests of a port of the plurality ofswitch ports that holds the token have highest priority, and wherein thetoken position counter is incremented at a predetermined interval oftime, wherein access is denied to a switch port of the plurality ofswitch ports that possess the fairness token until local requests forthe port can be serviced.
 6. An apparatus to implement a process forinsuring fairness in a switch having a plurality of switch ports, theapparatus comprising: assignment means for assigning each switch port apriority level for use in arbitration between conflicting connectrequests from multiple switch ports to an identified destination port;circulating means for circulating a fairness token among said switchports so that each switch port receives said fairness token, holds itfor a time and then forwards it to another switch port; wherein theassigning means further assigns a switch port that has said fairnesstoken in its possession a highest one of said assigned priority levels,wherein each switch port is coupled to a fibre channel arbitrated loop(FCAL) net having one or more nodes coupled thereto, and wherein accessis denied to a switch port of the plurality of switch ports that possessthe fairness token until local requests for the port can be serviced. 7.A computer-readable medium, encoded with computer code for causing acomputer to execute a process to insure fairness in an switch having aplurality of switch ports, the process comprising: assigning each switchport a priority level for use in arbitration between conflicting connectrequests from multiple switch ports to an identified destination port;circulating a fairness token among said switch ports so that each switchport receives said fairness token, holds it for a time and then forwardsit to another switch port; assigning a switch port that has saidfairness token in its possession a highest one of said assigned prioritylevels; and denying access to a switch port of the plurality of switchports that possesses the fairness token until local requests for theport can be serviced, wherein each switch port is coupled to a fibrechannel arbitrated loop (FCAL) net having one or more nodes coupledthereto.
 8. The computer readable medium of claim 7, wherein the processfurther comprises: keeping a camp list for busy ports which havereceived multiple connect requests from different switch ports; when aswitch port in possession of said fairness token makes a connect requestto a switch port with a camp list which is not full, adding the ID ofsaid switch port having said fairness token to said camp list; whenaccess to said switch port with the camp list is granted to the switchport in possession of the fairness token, forwarding said fairness tokento another switch port.